Crack the egg into a small mixing bowl.
Add the Greek yogurt and sweetener.
Mix until smooth and fully combined.
Add cocoa powder and chocolate protein powder.
Stir until no dry spots remain in the batter.
Fold in chocolate chips if using.
Lightly grease a microwave-safe mug or ramekin.
Pour the batter into the prepared dish.
Microwave for 90 seconds for a soft, gooey center.
Microwave closer to 2 minutes if you prefer a fully cooked brownie.
Allow to cool for 1 minute before eating.
Add optional toppings if desired.