Step 1

Add the butter to your mug and microwave until melted (about 30 seconds).

Step 2

Whisk in the milk, vanilla extract, egg yolk & sugar with a fork.

Step 3

Then whisk in the flour and baking powder. Stir in the sprinkles.

Step 4

Microwave for 60-90 seconds on medium power. The top should look set when it's done.

Step 5

Cool for at least 1 minute before enjoying.

Step 6

Beat the butter using an electric mixer or whisk with a fork until smooth.

Step 7

Beat/whisk in the powdered sugar 1 tablespoon at a time until the desired sweetness is reached. Then beat/whisk in the milk.

Step 8

Spread/pipe on top of the cake.