In a microwave safe mug, mix together all of the cake ingredients except the sprinkles until fully combined. Fold in 1 teaspoon sprinkles. Sprinkle the rest on top.
Microwave for around 1 minute, or until the cake looks done. Allow to cool for around 1 minute.
Meanwhile, mix together the frosting ingredients except the sprinkles until smooth. Pipe on top of cooled cake and top with additional sprinkles, if desired.
Notes
The frosting makes quite a lot. You could probably use this amount of frosting for 2 mug cakes.
Make sure your mug is a good size. The cake will expand in the microwave then deflate. If your mug is too small, the cake could overflow.