Bullet Proof Coffee ("Fat Coffee") Fasting

I’m experimenting with fasting for the last 8 pounds (seems so little and turns out to be soooo stubborn) and it works surprisingly well for an 18 hour fast in terms of being comfortable.I know I can I have normal coffee during this time.
Any experience with coffee and butter? I know a dash of cream is fine, is there a caloric limit for butter?
Is consuming fat, in general, neutral when it comes to insulin reaction?