Protein, electrolytes and proper diet. Not supplements but eating well (and eating enough) in your window. How long have you been at it for? There is a period of adaptation, like 1-2 weeks, which I found pretty miserable. Fatigue and hunger pangs. But you adapt. Also, maybe reduce the hours you fast and work up to a longer periods. Try 14:10 for a few weeks to ease yourself in. Then move to 16:8. If 14:10 is miserable then 12:12 (e.g. 8am - 8pm) is still better than nothing and you can gradually have later breakfast or earlier dinner. If you do 12:12 be more careful about calorie counting.
If you’re already doing short fasts, there is a risk that the bulletproof coffee is actually undermining you a little - in the morning, while I assume you are fasting, you introduce a shot of calories to your system. It knocks you out of a fasted state and then your stomach/body is like, oh, time to get ready to eat! And then no food comes. I get hungrier and more tired if I’ve had a little bit of food, or smell food, and then don’t eat, otherwise I manage to self regulate pretty well. You just have to cut yourself off.