Mix all ingredients into a microwave-safe mug.
Stir (I used a fork) until all ingredients are thoroughly blended in the batter, especially the Nutella.
Microwave for approximately 1 min 15 seconds. Check on the cake. The top of the cake may still look wet, but check if the inside of the cake is set and cooked. If it is, it's done! Careful not to overcook or the batter will get too dry. Total cook time should be around 1.5 minutes, and no more than 2 minutes.
Wait for the cake to cool before eating (about 10 minutes).