In a large microwave-safe mug, combine oat flour, protein powder, cocoa powder, baking powder, and sweetener.
Pour in the milk and melted butter; stir until the mixture is smooth. If it seems too thick, add another splash of milk. If using, sprinkle chocolate chips on top.
Microwave on high for 60 to 90 seconds. Start checking at 60 seconds and continue every 10 seconds until the cake is cooked through.