Milk-free ... Ruby's Diner serves up a paleo butter coffee for $8. Photo: Supplied

Haven't you heard? Butter in your coffee. It's the latest health fad spruiked by fans of the paleo diet, and those looking for a mental edge. And it's available in Australia, for $8 a cup.

Butter-coffee may sound a little wacky, gross even, but fans say it tastes better than a creamy latte.

Advocates use one-to-two tablespoons of butter (or ghee) blended into their coffee in place of milk - and perhaps a spoon of coconut oil for good measure. It's the ultimate heart-starter - or heart-stopper - depending on where you fall in the good fat, bad fat debate.

Dave Asprey has made butter coffee - and bringing this to a wider audience - his business. Asprey is a US-based health entrepreneur whose website The Bulletproof Executive and accompanying blog brings to mind an episode of Bear Grylls' Man versus Wild. Asprey stumbled upon the concept of a fatty, caffeinated morning beverage while trekking in Tibet - where tea made with yak butter is a staple. His particular twist is a trademark brew called Bulletproof Coffee - or "highest performance coffee" – and uses grass-fed, unsalted butter and an Asprey product called Bulletproof Upgraded Brain Octane (coconut fat derived). Blended together with black coffee for 20-30 seconds, the result is a frothy creamy brew.

Melbourne barista (and former World Barista Champion) Matt Perger of St Ali began experimenting with butter and ghee in coffee about a year ago. It's not on the menu at St Ali's Australian cafes but Perger says he and his girlfriend make it at home and he believes it has big potential for the cafe crowd. "Perhaps we'll start serving it soon with paleo-oriented breakfasts like steak and egg."

"When made well it's actually quite a delicious coffee and also provides you with a lot of calories early in the day from the saturated fat. I'm not a dietitian ... but it keeps you fuller for longer."

Not on the menu yet, but St Ali's Matt Perger does make butter coffee at home. Photo: Eddie Jim

Sydney cafe Ruby's Diner has jumped on the butter coffee bandwagon, selling it as "the paleo answer to coffee". Owner Ed Devlin began selling Bulletproof coffee about eight weeks ago. The paleo-friendly cafe charges $8 and interest is slowly picking up. "We probably sold about 30 on the weekend," Devlin says.

The cafe uses Asprey's refined coconut oil product, but their own choice of coffee - a single origin from Proud Mary. He described most people who have tried the brew as "health nuts".

Devlin says he believes claims that the "high quality fats" improved brain power and prevented a caffeine "crash". "You don't get as jittery - the caffeine high is more even and lasts for longer ... I used to drink five or six coffees a day, and for the past five weeks I've just been having one Bulletproof coffee a day."

However nutritionist Dr Rosemary Stanton warns people not to take on board the health claims of those spruiking butter or ghee coffee.

"Such claims have no basis in fact," she said. "Arguments that saturated fats - even from grass-fed cows (which is pretty well all dairy cows in Australia) - are somehow healthy is simply not supported by evidence," she says.

"The fad is common among some paleo enthusiasts who won't drink milk but will eat butter or ghee that are made from milk. That doesn't make sense ... It's also promoted by distributors of one brand of coffee in the US.

"Although butter is added to tea in places like Nepal and Tibet, these people have every opportunity to use up the kilojoules. Most sedentary Westerners don't."

But local grass-fed butter and ghee makers are already tapping into the fatty coffee craze. Sydney-based artisan butter company Pepe Saya's ghee is recommended to customers of Optimoz, an Australian website that sells Dave Asprey's products.

Pepe Saya's owner Pierre Issa says sales of his ghee have taken off since Optimoz became a customer about a year ago. "They are by far our biggest buyer," says Issa. Issa says ghee is particularly popular with people who cannot tolerate lactose, because milk solids are removed to make ghee.

Another beneficiary is the Myrtleford Butter Factory, which has seen a spike in customers asking for "butter for coffee", according to director Naomi Ingleton. "We have been doing butter for coffee for about a year now. Most of our mail order customers are buying unsalted butter by the kilogram ... We have quite a few customers, mainly males aged between 30 and 50 years." Ingleton says the company is looking into selling butter for coffee to cafes.

Perger says health debates aside, butter coffee - like most things made with oodles of fat - simply tastes good. "When fresh and hot and just whisked [butter coffee] just tastes like a latte made with high-fat milk.

"It has a nutty, chocolatey taste. There's so much fat in it, it blocks a lot of your taste receptors so the coffee is less bitter."

Perger recommends blending or whisking 250ml of filter coffee (or a single-shot espresso diluted with 100ml water, or double-shot diluted with 200ml water) with 15 grams of unsalted butter and 15 grams of extra-virgin coconut oil. Using enough water is essential says Perger, otherwise you'll end up with something akin to mayonnaise that has split.