Add flour, sugar, baking powder, and salt to a mug and stir together.
Stir in milk, melted butter, and vanilla extract until smooth, being sure to scrape the bottom of the mug. Stir in sprinkles.
Cook in microwave for 70-90 seconds* (until cake is just set, but still barely shiny on top). Allow to rest in microwave for 1 minute before consuming.
*Because all microwaves are different, it's difficult to give a precise cook time. Start with 70 seconds, wait 1 minute while it cools, then look to see if it is set. Add an additional 15-25 seconds if needed. Allow it to rest in the microwave for 1 minute.• The trick with mug cakes is to stop cooking the cake earlier than you think, because you want it to be soft on the inside, and the cake will also continue to cook as it cools.