Gather ingredients: Preheat oven to 350°F.

Make cake batter: Prepare confetti cake mix according to package directions. Divide batter evenly among 3 (6-inch) round cake pans coated with baking spray.

Bake and cool cake layers: Bake in preheated oven until a wooden pick inserted in centers of cakes comes out clean, 20 to 23 minutes. Cool in pans 10 minutes. Invert cakes onto wire racks; cool completely, about 45 minutes.

Make frosting: Beat softened butter in a large bowl with an electric mixer on medium speed until smooth, about 2 minutes. With mixer on low speed, gradually add powdered sugar, sour cream, and vanilla extract. Beat until fluffy, stopping occasionally to scrape down sides of bowl, about 3 minutes. (You will have about 5 cups frosting.) Set aside about 1 cup of frosting in a separate bowl, and fold in rainbow candy sprinkles.

Assemble cake: Place a cake plate on a stand or a lazy Susan. Put 1 cake layer on plate. Spread 1 cup plain frosting over top, smoothing top using a bench scraper or an offset spatula. Repeat with remaining 2 cake layers and 2 cups of the frosting. Using a bench scraper or large offset spatula, spread a thin layer of frosting (about ½ cup) over sides of cake. Chill until frosting has firmed and set, about 30 minutes.

Add sprinkle frosting to outside of cake: Spread a thicker layer of reserved sprinkle frosting (about 1 cup) on sides of cake, smoothing as you go.

Decorate cake: Spoon any remaining frosting into a piping bag fitted with a medium round tip. Pipe dollops around top edge of cake. Garnish with additional sprinkles in center of cake. See our step-by-step instructions for frosting a smooth-sided cake with sprinkles here .