Mix wet ingredients: In a large microwave-safe mug, add the egg, milk, oil, and Nutella. Whisk well until smooth.
Add dry ingredients: Add flour, cocoa powder, sugar, baking powder, and a pinch of salt. Stir until the batter is fully combined.
Microwave: Place the mug in the microwave and cook for about 1 minute and 30 seconds to 2 minutes (depending on your microwave power).
Check texture: The cake should rise and still be slightly gooey in the center. If it’s too liquid, microwave for 10–15 seconds more.
Serve: Let cool for 1–2 minutes, then top with a spoon of Nutella, whipped cream, or powdered sugar. Enjoy warm!