How to Make It
Prepare cake layers according to recipe or package directions.
Prepare the Frosting: Beat together butter, vanilla extract, and salt with an electric mixer on medium speed until creamy, about 1 minute. Gradually add powdered sugar, beating until fluffy, about 3 to 4 minutes. Add milk, 1 tablespoon at a time, until frosting reaches a smooth, spreadable consistency.
Spread frosting between layers and on top and sides of cake, to create a crumb layer.
Divide remaining frosting among 5 small bowls. Using 1 color per bowl, add 1 to 2 drops color into each bowl. (We used yellow, pink, purple, orange, and blue.)
Place a few dollops of each color frosting randomly on sides and top of cake. Use an offset spatula to smooth frosting around cake. (*Be sure not to use too many strokes, or the colors will look muddy.)
If desired, combine 2 complementary frosting colors in a piping bag, and pipe a design around edges of cake.