1. In a bowl, combine the flour, unsweetened cocoa powder, and granulated sugar. Mix well using a fork. Add the egg and milk, then mix again using a fork. Once the mixture is well combined, melt the butter and add it to the mixture. Mix again.
2. You can now add the Nutella, then mix thoroughly to obtain a smooth and creamy batter.
3. Grease 2 cups or 2 mugs (make sure to use microwave-safe cups), then pour the batter into them, filling them up to 3/4 full.
4. Microwave them on high power for 1 minute and 30 seconds... And enjoy immediately!