Use a large microwave-safe mug, at least 350 ml (12 oz), as the cake rises a lot.
Add the protein powder, oat flour, cocoa, baking powder and sweetener to the mug and stir with a fork.
Add the egg and milk and whisk with the fork for 30 seconds until completely smooth, scraping the bottom edges.
Push the chopped chocolate into the batter so it melts into pockets.
Microwave on high for 60 to 75 seconds (based on a 1000 W microwave; add 10 to 15 seconds for 700 W) until risen and just set on top. A slightly wet centre is perfect; protein cakes turn rubbery if overcooked.
Let it stand 1 minute, as the mug and cake are very hot, then eat from the mug.
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Tips
Overcooked protein cake turns rubbery: stop as soon as it rises and looks set at the edges.