In a microwave-safe mug, mix the protein powder, flour, cocoa powder (if using), baking powder, sweetener, and salt until well combined.

Add the egg, milk, vanilla extract, and melted coconut oil or butter to the dry ingredients in the mug.

Stir thoroughly until the mixture forms a smooth batter with no lumps.

Microwave the mug on high for 60-90 seconds, or until the cake has risen and is cooked through. Be careful not to overcook as the cake can become dry.

Allow the mug cake to cool for 1-2 minutes before enjoying. Optionally, top with fruits, nut butter, or yogurt for extra flavor.