Combine flour, sugar and cocoa powder in a large microwave-safe mug. Then add the egg, milk, oil, vanilla extract and Nutella. Whisk well with a fork until smooth.
Microwave on high for 1.30 - 3 minutes (time depends on microwave wattage). Check it at 1.30 minutes to see if it is done or not. If not, continue to microwave, stopping every 30 seconds to check. Mine took 2.30 minutes.
The chocolate cake will rise over the top of the mug and then sink down when the microwave is turned off. Remove from the microwave (be careful, it's hot) and serve immediately.