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This, Aga Mis, the manager of the Victoria branch tells me, is 15 times stronger than the regular brew.

“We started doing it about four or five months ago. It is getting a lot more popular. It is the sort of drink that you develop a taste for. We sell about 10 a day.”

Such concoctions, say the critics, encourage the worst in modern-day narcissism and will no sooner help you lose weight than a bacon butty. Incidentally, Asprey preaches an overall diet rich in saturated fat to be enjoyed alongside his coffee.

‘I love butter. And I love coffee. But definitely not the two together’

“There is no science that would back this up as a weight-loss tool,” says Dr. Sally Norton, an NHS surgeon and leading weight-loss consultant in the U.K. “The problem is we have a huge obesity epidemic where two thirds of us are overweight and one quarter obese. The more we try to lose weight, the worse it is getting. People are just desperate and don’t know where else to turn.”

Sian Porter, a spokeswoman for the British Dietetic Association, who first heard about fat blacks a few weeks ago, is similarly skeptical. “He has done one of the classic things that fad diets do by saying he has looked at thousands of studies. But it is about quality not quantity.”

As for the taste of a buttery coffee, and the claim that the drink boosts “mental clarity,” who better to ask than the harried office workers of London walking the streets during a week of Christmas parties?

“All you can taste is the butter,” says Stuart Butcher, 40, who works in acquisitions and marketing. “Normally, I would just go for an Americano, maybe with a bit of cold milk.”

Craig Timmis, a 33-year-old lawyer, decided to have a sip on his way back from lunch at a nearby Thai restaurant. “It tastes very weak, but there is a greasy film that definitely stays with you.”

“I love butter, I put it on everything, and I love coffee as well,” says Helena Kilduff, 26, an account manager.

“But,” she says, regarding the droplets of oil formed on the surface, “definitely not the two together.”

Next year, Asprey plans to open his first Bulletproof café in Los Angeles. Until then, viva the milk and two sugars, and save the butter for your toast.