Mix protein powder, baking powder, cocoa powder, and sweetener in a microwave-safe mug.
Stir in the egg until smooth, then add a little milk to thin the batter if needed. Fold in chocolate chips if using.
Microwave for 30–40 seconds, adding 5 seconds at a time until fluffy.
Let cool slightly, then enjoy. 
Notes: For a lava-style centre, stop cooking while the middle is still slightly soft. Serve with yoghurt, nut butter, berries, or a light dusting of cocoa powder. Best enjoyed fresh for optimal texture and flavour.