... is actually pretty good.
One of our sons and his girlfriend are trying the Bulletproof Diet. I haven't looked it up, but I'm sure it's out there somewhere on the Interweb Tubes. You can only eat from 2 PM to 8 PM, but you can start your day with buttered coffee.
Lunatic diets aside, the coffee is tasty. We've got a Keurig which, after trying just about every cup I can get my hands on, produces coffee only a step up from instant. It can, however, heat water for a French Press.
The buttered coffee can be made easily in the French Press. You take a teaspoon of butter, put it on the bottom of the press and then put your coffee grounds on top of that. You then convince the Keurig to put the appropriate amount of hot water into the press and proceed normally. The butter melts, but doesn't form a layer at the top. Maybe it gets smushed into tiny particles by the press.
In any case, the resulting concoction is delish! Try it sometime.