In a 10 oz microwave safe mug, add in the butter and microwave until just melted, about 30 seconds.

Whisk in the milk, egg yolk, vanilla extract and sugar until well combined.

Add in the flour, salt, baking powder and sprinkles. Mix until just combined, be sure to scrape the bottom of the mug well.

Microwave for 45 seconds. If more than a ½” circle in the center is still wet, microwave for another 10-15 seconds. If you use a taller mug it may require more cooking time, microwave in 10 second increments until just set. Let cool in the microwave for 1 minute. Enjoy.

To make the frosting: Beat the softened butter with a hand mixer or whisk until smooth, beat in the powdered sugar one tablespoon at a time, adding drops of milk until desired consistency and level of sweetness is reached, add in ¼ teaspoon vanilla extract and beat briefly. Color with food coloring of choice if desired.