In a mug, combine the dry ingredients (all-purpose flour, granulated sugar, baking powder, salt).
Add an egg, melted butter, milk, and vanilla extract. Whisk with fork or a mini whisk to combine.
Add sprinkles to the batter. Mix together.
Microwave for 60-90 seconds until the cake rises and sets. Let the mug cake rest for 30 seconds in the microwave. Then, top with more sprinkles and enjoy!