In a small bowl, whisk together the flour, cocoa powder, sugar, and baking powder.
Stir in the milk, melted butter, and vanilla just until combined.
Pour the cake batter into a large microwave-safe mug. Microwave on high for 1 minute and then check for doneness. A toothpick inserted into the center should come out clean. Continue microwaving in 30-second intervals if needed until the cake is done.
Cool slightly then add the topping of your choice and enjoy.