https://www.bulletproof.com/
Personally I think Bulletproof is overpriced, highly marketed coffee and MCT supplements that you can buy off the shelf at Save On for half the price, but I know lots of people that swear by it.
By putting butter and MCT in your coffee all you are doing is adding calories so you don't waste away while fasting. I have a hard time believing it is doing anything over and above that.