Place a microwave-safe mug on the counter. Add 4 tbsp all-purpose flour, 1 tbsp cocoa powder, ¼ tsp baking powder, and ¼ tsp salt; stir with a spoon to combine the dry ingredients evenly.

Add 3 ½ tbsp milk, 1 ½ tbsp vegetable oil, ½ tsp vanilla extract, and 3 tbsp Nutella to the mug. Stir thoroughly until the batter is smooth and there are no dry streaks of flour. Scrape the sides and bottom of the mug as needed.

Smooth the top of the batter with the spoon so it is level.

Microwave the mug on high for 45 seconds. After 45 seconds, check the surface: if it still looks wet, glossy, or you see loose batter, microwave for an additional 15 seconds.

Let the cake cool in the mug for 1–2 minutes (careful, it will be hot), then enjoy.