1. In a mug, add in an egg, milk, vegetable oil, and Nutella. Stir until combined.
2. Sift in flour and cocoa powder. Add in sugar and baking powder. And stir until well mixed.
3. Microwave for 1 minute. The power of individual oven is different, so be careful. Then repeat microwaving for 10 seconds only until the cake is done.