Crumble half the prepared and cooled cake into a large bowl. Add 1/4 tub frosting and 2 tablespoons sprinkles. Stir until combined.
Divide dough equally in separate bowls to be colored. I divided mine into four different colors, but you could do two colors and just divide it in half. Add a few drops to each dough ball and mix in until you've reached your desired color and it's uniform throughout. Repeat for each remaining color; set cake balls aside.
Preheat oven to 350 degrees. Line regular-sized muffin pans with paper liners; set aside. In large mixing bowl, blend cake mix, sour cream, canola, eggs, vanilla on low until mixed.
Spoon a level tablespoon of batter into each muffin cup, placing a prepared cake ball on top. Cover cake ball with another tablespoon of batter. Bake about 18 minutes or until toothpick inserted near edge comes out clean. Cool completely.
In large mixing bowl, beat butter until creamy. Beat inthe powdered sugar, vanilla andmilk, adding milk if needed to reach desired piping consistency. Pipe frosting onto cupcakes. Decorate with additional sprinkles, if desired.