Preheat oven to 180ºC (360ºF) or air fryer to 160ºC (320ºF).
In a bowl stir together Greek yogurt, cocoa powder, granulated sweetener, coffee powder and pinch salt. Mix well until all is combined.
Stir in protein powder, oat flour and baking powder and mix until combined - if you make this brownie with vegan protein powder or casein you may need 1 to 2 tsp of milk to fix the dough.
Transfer protein brownie batter to a greased skillet or ramekin and top with chocolate chips.
Bake in hot oven for 10 to 12 minutes or air fry for 8 to 10 minutes.
Serve warm and top with (sugar-free) vanilla ice cream if you want.