Add the flour, sugar, baking powder, and sprinkles to a microwave safe mug (8oz. or larger). Stir until everything is well combined.
Add the milk, melted butter, and vanilla extract to the mug. Stir until a soft batter forms, making sure to scrape all the flour out of the corners of the mug.
Microwave the mug cake for about 60 seconds, or until it has puffed to about 2-3 times the volume. Cook time will vary with the wattage of your microwave (I used a 1000 watt microwave and 60-70 seconds was perfect).
Allow the mug cake to cool for a few minutes and then top with your preferred frosting option, or enjoy unfrosted.