What is a Bulletproof coffee?
The general rule is that any drink with sweeteners of any kind, or any fats of any kind can break your fast.
Basically, it has to be an unsweetened, black coffee; any coffee beverage other than that will likely break your fast.
I think sweeteners could activate a cephalic insulin response in some people. The insulin release would trigger the body to utilize glucose from the blood stream thereby breaking a fast.