Heat oven to 350 degrees F. Spray three 6-count donut pans with cooking spray.
Whisk together cake mix, sour cream, melted butter and egg in a large bowl until well blended. Spoon mixture into a resealable food storage bag and cut off a corner.
Pipe into donut pan, filling no more than halfway up the sides. Bake for 10 to 12 minutes.
Remove from pan immediately and cool completely on a wire rack, about 15 minutes.
Whisk together confectioners’ sugar, melted butter, 2 tablespoons of milk, and a few drops food coloring, if using. Add more milk to thin, if needed.
Once donuts are cooled, spoon glaze evenly over the top of each donut. Immediately decorate with sprinkles, if desired. Let frosting set before serving, about 5 minutes.
Before spooning donut batter into food storage bag, stir in 1/2 cup sprinkles or mini chocolate morsels to customize these birthday cake donuts.
If you don’t have 3 donut pans, bake 1 batch of donuts, remove from pan, wash pans and spray with more baking spray before baking more donuts.
For a marbled glaze appearance, add 2 to 3 very small drops of food coloring to white glaze and mix briefly, just enough to make a few streaks of color. Then use a spoon to ladle glaze over the donuts and decorate as desired.