Sarah Silverman tosses her long black hair and bares her perfect teeth. She steps up to the microphone, demure as a schoolgirl on the witness stand. The crowd tenses. 'I don't want to belittle the events of September 11 - they were devastating. They were beyond devastating....' She stops; words can't express her pain. 'I don't want to say especially for these people, or especially for those people but... especially for me.'

She lowers her sleek eyebrows. The audience in the little North Hollywood theatre takes a nervous breath. Her voice gets gritty: 'Because it happened to be the same exact day that I found out that the soy chai latte was, like, 900 calories.' There's a titter. I'm thinking - you can't do this in America? Can you? 'I had been drinking them every day.' She shakes her head. 'You know, you hear soy, you think healthy... And it's a lie.' Words fail her. Pause. Regather. 'It was also the day we were attacked...' The audience pauses, on the brink: they could go one way or another. Then the laugh comes.

Silverman has been booed off stage for this gag, but here on the West Coast this stuff works. You can sense the crowd patting itself on the back at its unshockability. So she pushes them a bit further: 'Positive spin! Take something tragic and make it something good! If American Airlines were smart their slogan would be: American Airlines. First through the towers.'

I was raped by a doctor... Which is so bittersweet for a Jewish girl.

Her default persona is goofy Jewish American Princess. It makes the material all the more shocking: no one so dopey-cute and girl-next-door has ever been so nasty. She does Holocaust jokes, Martin Luther King jokes, Aids jokes and lots on Jewish girls and sex - chiefly anal sex and abortion. If she has a staple comedic form, it's obscenity delivered with a cosy flirtatiousness. 'Last night I was licking jelly off my boyfriend's penis' - big kooky smile - 'and all of a sudden I thought - Oh my God! I'm turning into my mother!'

Her first broadcast gag was about the US legislation imposing a 24-hour cooling-off period after someone seeks to terminate a pregnancy. 'I think it's a good law,' she said. 'The other day I really, really wanted to get an abortion. I totally did. But then I thought about it and it turned out I was just thirsty...'

Those teeth work hard for her act: the super-clean smile takes the toxic edge off the jokes. With each gag that the audience masters, she makes it a little harder, like you train a dog to do tricks.

Many Silverman fans (and the people who think she is the best thing happening today in American comedy include a range of slightly off-centre stars, from Moby to Mandy Moore and Jack Black) hold up one song she does to explain their love for her. It shows just how far she'll go into the dangerous swamps where irony meets racism - and what fun she has splashing around there. It starts like this:

'I love you more than bears love honey, I love you more than Jews love money, I love you more than Asians are good at math, I love you more than black guys don't tip...'

You can find a clip on the net of her doing the song, strolling across a car park in a mini-dress strumming her guitar like a pop video from 1982. Just as she blithely delivers the line: 'Maybe it's like when black guys call each other niggers...' she finds herself face-to-face with two big, unsmiling black guys. The guitar stops, her grin drops, they stare her out, she looks like she's going to cry, they start to laugh, she laughs too, they stop laughing and fix her with the stare again. She crumbles. It's an epic of white urban angst encapsulated in 20 seconds.

I don't care if you think I'm racist, I just want you to think I'm thin.

Silverman's big break came as a result of America's problem with defining what racism actually is. The story's a good a lesson, too, in the rule that if you want to get on in showbiz, you might consider getting into trouble. It's 2001. Sarah Silverman is an LA circuit comic trying to get a big break. She's known for being enterprisingly obscene. She's 30 and has been on the scene over a decade, doing OK: lots of stand-up, morsels of TV work (Seinfeld, The Larry Sanders Show, Star Trek: Voyager) some bit parts in good movies (The Way of the Gun, There's Something About Mary). Usually she plays the supporting star's girlfriend. Her real-life boyfriends are always comics and usually more famous than her. But because she's cute and talks dirty she gets regular guest spots on late-evening network TV talk shows - she makes those middle-aged male presenters blush.

So in July 2001 she's doing a third-guest spot on Conan O'Brien's show. She tells how she got selected for jury duty, which was a pain - she didn't want to do it. 'So I'm filling out the form. And my friend said, "Why don't you just write something really inappropriate, like I hate Chinks?" And I said, "Yeah, that's a good idea." But I don't want people to think that of me, you know? I just wanna get out of jury duty. So I just filled out the form, and I wrote, "I love Chinks." And who doesn't?'

O'Brien buried his head in his hands but rode over it. However, within days the NBC network, inundated by complaints, had to make an abject apology. As the row brewed, Silverman appeared on a televised face-off with Guy Aoki, the chair of an Asian-American lobby group which maintained that the 'racial slur' had offended millions of people. She said that the joke was not racist, but about racism, and then, unusually, lost her cool and called Aoki 'a douchebag'. Aoki was generally judged to have won the encounter: Silverman says she will never again make the mistake of defending her material.

Though Silverman came off worse in that run-in with the PC nation, the episode gave her career a crucial push forward. Suddenly, in the welter of LA wannabes, she had an identity. She was the Jewish comic who does racism. And she was an underdog liberal, bullied by the politically correct. In the blog world and in the media Sarah Silverman was suddenly a player in America's great cultural war.

Naturally enough, the episode became a part of the stage act. She tells the North Hollywood audience how she got into trouble for saying the word 'Chink' on a talk show and how the Asian-American watchdogs called her a racist and it was reported everywhere. 'It hurt. Yeah. As a Jew - as a member of the Jewish community - I was really concerned that we were losing control of the media. Right?'

She became a regular on the cable channel Comedy Central. Better movie parts arrived - like in the 2003 hit School of Rock: sure, she's still playing girlfriends (Jack Black's room-mate's here), but she has a real (and nasty) character. Silverman performed on last year's documentary The Aristocrats, which watched comedians telling the legendary, obscene gag of that name that has for decades been a secret handshake among comics. Silverman's version was the rawest, not least because in it she announces, in the tone of someone 'recovering' a traumatic memory in therapy, that a grand old showbiz figure, Joe Franklin (think Jimmy Savile), molested her as a child. Franklin subsequently threatened to sue.

Conscious of the gains to be had from offending Middle America, her next touring stage show was titled 'Jesus is Magic' and the mainstream papers - never much interested in stand-up - came to review her. 'Sarah Silverman is the funniest woman alive!' announced Rolling Stone. The LA Times and the other critics could find only one comedian for comparison - the tragic Sixties baiter of the bourgeoisie, Lenny Bruce, who was driven to suicide after being persecuted by the authorities for his 'obscene' stage show. Silverman started getting better work - suddenly she was a comedian's comedian, in the club.

If my boyfriend [who's a Catholic] and I ever have a kid, we'll just be honest with it. We'll say that Mommy is one of God's chosen people, and Daddy believes that Jesus is magic!

Last November a feature-film version of Jesus is Magic, featuring sketches and footage from that North Hollywood live show of 2004, got a limited release in the States (not in the Bible Belt) and some acclaim in the liberal media. Entertainment Weekly put her on its 'Entertainers of the Year' list and Rolling Stone put her on its cover. Profiles and essays resulted in publications as august as The New Yorker and The American Prospect.

What the movie does reveal is two distinct comedians: a navigator of highly complex landscapes of irony who is also a gross-out, bottom-burp-loving comic to rival Adrian Edmondson or the Little Britain team. The Sarah Silverman who has essayists hailing her experiments with taboo as post-modern performance art is the same Sarah Silverman who closes her stage act with a rousing Amazing Grace in three-part harmony - with three microphones. The first is at her mouth, the second at her groin. Her bum sings the bass part.

Among the more solemn dissections of her act, one stood out as quite sensible. Sam Anderson, writing for online magazine Slate, announced a label for her (along with Ali G and the kids from South Park): meta-bigot. 'Meta-bigots work at social problems indirectly; instead of discussing race, rape, abortion, incest, or mass starvation, they parody our discussions of them. They manipulate stereotypes about stereotypes. It's a dangerous game: if you're humourless, distracted, or even just inordinately history-conscious, meta-bigotry can look suspiciously like actual bigotry.'

I dated a guy who was half-black, but he totally dumped me because I'm such a loser. Wow! I just heard myself say that. I am such a pessimist... He's actually half-white.

Silverman is the third of four sisters from a liberal, middle-class New Hampshire Jewish family. Her mother was a drama teacher and encouraged her to perform: her father, who had a clothing shop chain, taught her off-colour jokes. ('My dad is one of those dads who thought it was hilarious to teach his daughter swear words, you know. The first thing I said was "bitch, bastard, damn, shit" - that was what my dad taught me... I got such a positive reaction at such an early age!') One of her sisters became a rabbi, and wrote a book called Jewish Family & Life: Traditions, Holidays and Values for Today's Parents and Children. Her parents divorced when she was seven.

Sarah was a teenage bed-wetter - she announces this gleefully in her act - and she is pretty open too about the acute panic attacks she used to suffer. She did some serious drugs, too, as a teenager and in her early twenties. She's still a four-times-a-week pot smoker and, since she was 24, 11 years ago, she has never been off anti-depressants.

As a schoolgirl she did am-dram - she played the title role in Annie - and watched all the comedy she could. 'I love Steve Martin' was painted on her bedroom ceiling. She started doing stand-up gigs in her teens - her family remembers a performance at a local Mexican restaurant whose highlight was a song 'Mammaries', to the tune of 'Memories', about her lack of breasts. She left New York University after just a year, a move sanctioned by her father, to pursue a life as a stand-up on the comedy circuit.

Three years later, in 1993 and aged 22, she had a big break - too early, perhaps, and too big. She was hired by the TV show Saturday Night Live, then, as now, American comedy's prime forcing-ground for new talent. She worked with Mike Myers and Adam Sandler, but her jokes rarely made the show and she was sacked. She moved to LA, and began the hard labour of TV bit-parts and gigs at the Hollywood Improv comedy club that eventually led to that night on Conan O'Brien.

Most American comics hail their British antecedents - the Pythons, Cleese, Benny Hill (always more popular beyond these shores), Rowan Atkinson. But Sarah's heroes are Woody Allen and Steve Martin. Asked to name women, she comes up with Joan Rivers, Paula Poundstone, Barbra Streisand and Ellen DeGeneres, but her 'all-time favourite funny woman, no contest' was the character actress Ruth Gordon, best known for the movie Harold and Maude, who died in 1985. The only Brits she admires are Dawn French and Jennifer Saunders.

Silverman has never been to Britain - she told me in an e-mail conversation we had that she's been studying Madonna to try and get the British accent right, should Jesus is Magic get a release here. 'I'm dying to go. Though I've been told I should pack a lunch...' If she comes, she's of course welcome to stay with us (my wife says I'm obsessed) and the first thing I want to show her is BBC3's godawful Tittybangbang, possibly the duffest women-comics' sketch show in history.

We'll watch the grim repetitive sketches about nymphomaniac pathologists having sex with corpses and a woman whose silicone-filled breasts leak and women who are fat (Silverman doesn't do fat-women jokes - it's the only place she won't go: 'Because all women feel like they are fat women inside, in America anyway. And also because fat women are seen so differently as fat. Like fat men still deserve love.') And then I will begin a short lecture - which I'm sure Sarah will find interesting - titled: What happened to women's comedy in Britain?

An opinion poll carried out last year by BMRB to list Britain's funniest women came up with 20, four of them dead (anyone for Hylda Baker?), and none under 40. The youngest was Caroline Aherne, co-writer and star of the late-Nineties hit, The Royle Family, who has done little since. Other than Aherne and the actress Kathy Burke, every woman on the list had peaked in the Eighties or earlier.

Again and again women stars seem to rise from stand-up, burn fast in TV comedy, then disappear. Pamela Stephenson was the first such example from the modern age; Doon Mackichan and Rebecca Front represented the best of groundbreaking Nineties TV comedy like The Day Today and its spin-offs. They were teamed with names like Steve Coogan, Patrick Marber, Armando Iannucci and Chris Morris - but of all of those, the women have progressed least. It's a long-standing curse - what happened to Connie Booth, Monty Python regular and co-writer and co-star of Fawlty Towers?

There was a golden age of the female stand-up which coincided with the latter years of Britain's only female prime minister. Women comics do well in politically charged times and, in the late Eighties and early Nineties, stand-ups driven by the fierce engines of feminism and anti-Toryism seemed for a moment to be at the cutting edge of humour in this country. 'There was a tiny crack and the doors opened,' says Jenny Eclair, perhaps the funniest and certainly the fiercest of the 'hairy armpit' wave, comics like Jo Brand, Jo Caulfield, Hattie Heyridge and Rhona Cameron, now more often found on the safer shores of Radio Four.

'We changed female comedy,' says Eclair, 'and for a while it was very modern and fashionable. It was our job and we did it well. But people get tired of that - "Jenny Eclair talking about periods and Jo Brand talking about fat" - and I got jaded of being this sour monkey jumping about increasingly desperately. Women's stand-up has never got out of the ghetto - there's no female Eddie Izzard, able to go beyond sexuality, boyfriends, periods and weight. Though I still think there's room for a great period joke...' Some other great funny women whose careers were born in that era have done better - but most comics would call Jennifer Saunders and Dawn French primarily character actors. Victoria Wood, who was a stand-up with a penchant for considerable obscenity, according to legend, is now best known for writing middle-aged TV sitcom with Dinnerladies and Acorn Antiques.

Stand-up comedy is even more a masculine domain than television. Women are a tiny part of the scene. For instance, the bill for March's Glasgow International Comedy Festival features four women solo acts among over 30 male ones. Some of the men, too, have been around a very long time - Arnold Brown, for instance, is on the bill: he won the Perrier Comedy Award when I was on the judging panel for it at the Edinburgh Festival 19 years ago. Only two solo women (Eclair and, in 2005, Laura Solon) have ever won the Perrier in its 25-year history. If anything, women stand-ups are becoming rarer - in the nine years before Solon's victory, 36 men and only one woman were nominated.

Women's short shelf-life in comedy is in part explained by the fast track offered by TV to female talent - any woman whose head rises over the parapet of 'actually quite funny' on the stand-up circuit is instantly nabbed by TV execs desperate to improve the gender balance of their comedy line-ups. They don't often come back. Comedy critic Bruce Dessau deplores the trend, but can understand it. 'TV is a safe haven, there's a guaranteed paycheck, and lots of stand-ups have to take it. There's two female comics in the cast of Coronation Street at the moment.'

With TV taking the cream, says Dessau, the material done by women on the stand-up circuit is 'pretty safe and bland'. (Jenny Eclair is ruder: 'I think it's become very light, very frothy - very pixyish.') Dessau sees a few bright sparks - Shazia Mirza, for instance. But generally men are taking the form forward. And what I've seen on the stand-up circuit backs that up. There are a lot of women trying to out-bloke the blokes, with gross-out comedy the model - try Debra-Jane Appleby. There's some good work in the 'my life' story-telling genre, exemplified by Glasgow landlady Janey Godley. Some of the funniest traditional, free-form stand-up comes from visiting Australians like Kitty Flanagan.

Men outnumber women similarly in stand-up in the States, but women seem to make more of a splash. This is part explained by the fact that American mainstream television is far less welcoming to potentially offensive comedy than it is in Britain - it won't take the risks. So American comics like Silverman and Elizabeth Cho, who want to do material about sex and drugs, stay on the circuit much longer, polishing and refining, dipping their toe into TV with bit-part work to pay their bills, but using DVD and now cable TV as a means of circulating their adult stage material more widely, just as Richard Pryor and Eddie Murphy did with their 'in-concert' videos before them.

Silverman has two advantages comics here perhaps don't - she performs to a nation still a little more innocent, still capable of being challenged by comedy. Her other is that she doesn't do characters - or rather she does only one character - the beyond-Sarah, the Frankenstein Sarah. 'I actually think of it as a character,' she says. 'I mean, I don't have a weird voice or always have to be angry. I'm completely myself on stage. I talk the way I talk and I move the way I move, but inside, I think she's more obnoxious, ignorant yet arrogant.'

Silverman achieves something no British female stand-up since Victoria Wood has done, which is to create a wholly feasible comic persona, a stage self, who remains intact from show to show. Male comics like Eddie Izzard do it all the time - female comics, it seems, aren't given the the chance.

I've got a long neck. It's the best of my many good features. Swan-like. It's six inches, when flaccid.

The key to the character is that it has nothing to do with realism. She is a monster. Silverman seems keen to make sure you realise that. Which is why Jesus is Magic, the movie version, has a framing device that pictures Sarah squarely as an egocentric freak with scarily schizo tendencies. It ends with a scene where Sarah, having finished the stage show, arrives in her dressing room. 'You're fucking amazing!' she breathes at herself, in a dressing-room mirror, and then starts to kiss her reflection. 'You're a star - and I'm a star-fucker!' Soon the two Sarahs are snogging so hard that you expect them to start undressing each other. It's a surprisingly raunchy moment. It's a way of saying: Laugh but don't touch; I'm weirder than you think. And watching it you realise that Silverman has put something back into contemporary comedy that it badly needed - fear.