For whatever it's worth, two tablespoons of butter on your coffee is 204 kcal according to Wolfram Alpha. Even Bulletproof says things like "Bulletproof Coffee is breakfast" and "curbing your hunger with food that keeps you full for hours. Bulletproof Coffee checks those boxes." which I read as saying that they think of it as (liquid) food. https://www.bulletproof.com/recipes/bulletproof-diet-recipes...
You're replacing a potentially less structured breakfast (I eat different breakfasts every day) with a single standardised one. If you buy Bulletproof's analysis, you might be doing something ketogenic which might be beneficial in some way. But you're not skipping a meal. Though there is a blurb on that page that says something like "you're having a meal without breaking your fast because [arbitrary definition we made up on the spot]".
For whatever it's worth, I wouldn't take health advice from a product sold with an origin story of "the founder was dangerously overweight and at acute risk of stroke or heart attack at the age of 30, so the biohacker in them decided it would be smart to go on a difficult mountain trek in Nepal".