When Jimmy Fallon handed the actress Shailene Woodley a mug of coffee blended with butter on “The Tonight Show” in October, she didn’t recoil. Instead she raved to the audience about the cup of saturated fat:

“It will change your life!”

“It’s the most delicious thing ever,” Mr. Fallon said. “But it’s actually good for you. It’s good for your brain.”

It seems these days everyone is a coffee evangelist, but there are perhaps no proselytizers more fervent than those of Bulletproof coffee, a creation of the technology entrepreneur and biohacker Dave Asprey.

The recipe — a riff on the yak butter tea Mr. Asprey found restorative while hiking in Tibet — calls for low-mold coffee beans; at least two tablespoons of unsalted butter (grass-fed, which is higher in Omega 3s and vitamins); and one to two tablespoons of medium-chain triglyceride (MCT) oil, a type of easily digestible fat. Mr. Asprey claims having the 450-plus-calorie cup of coffee instead of breakfast suppresses hunger, promotes weight loss and provides mental clarity.

“It’s a gateway drug for taking control of your own biology,” said the well-caffeinated Mr. Asprey. Consider the strength of the addiction: At a three-day Bulletproof conference in Pasadena, Calif., in September, 200 pounds of Kerrygold butter wasn’t enough for the 500 attendees. There was also a run on unsalted grass-fed butter at the nearest Whole Foods.