In a large microwave-safe mug, mix together the all-purpose flour, salt, and baking powder.
Add the milk and 3 tablespoons of Nutella to the mug. Stir until the mixture is smooth and free of lumps. If it's too thick, add another splash of milk and combine.
Microwave on high for 70-90 seconds. Check if the cake is set and dry on top. If not, microwave in additional 10-second intervals until done. Cooking time will vary on your microwave and mug size.
Serve warm, with a dollop of Nutella on top. Garnish with whipped cream or fresh fruit if desired.