Steps:

• Heat oven to 350°F. Grease and flour 2 (8-inch) round cake pans or spray with baking spray. Set aside.
• Combine flour, baking powder and 1/4 teaspoon salt in bowl; set aside.
• Beat sugar and 1 cup butter in another bowl at medium speed, scraping bowl occasionally, until creamy. Add eggs, 1 at a time, beating well after each addition. Add 1 tablespoon vanilla; mix well. Add flour mixture alternately with milk, beating at low speed after each addition, just until mixed.
• Divide batter evenly among 6 bowls with about 3/4 cup each. Tint cake batters red, orange, yellow, green, blue and purple. Spread red and orange batter into prepared pans. Bake 8-10 minutes or until toothpick inserted into center comes out clean. Cool 3 minutes; remove from pans to cooling rack. Cool completely. Meanwhile, wash pans; grease and flour pans. Repeat with remaining colors until all cake layers are baked.
• Beat 2 cups butter in medium bowl at medium speed until creamy. Gradually add powdered sugar and 1/4 teaspoon salt alternatley with whipping cream and 1 tablespoon vanilla, beating at low speed until well incorporated. Beat at medium speed until smooth and creamy. Remove 2 cups frosting; set aside. Divide remaining frosting evenly among 6 bowls with about 1/3 cup each. Tint frostings red, orange, yellow, green, blue and purple. Set aside.
• Place purple cake layer onto serving plate, bottom-side up. Frost top with 1/4 cup white frosting. Place blue cake layer over frosting; frost with 1/4 cup white frosting. Continuing adding green, yellow, orange and red layers in that order, frosting with 1/4 cup white frosting between each layer. Frost outside of cake with remaining white frosting. This layer of frosting will be thin and cake will be visible through frosting. Once frosted, refrigerate cake 30 minutes. (This is called a crumb coat and will prevent crumbs from interfering when piping rosettes.)
• Place each tinted frosting into individual resealable plastic food bag; push frosting to one corner. Cut off 1/2 inch from tip of corner. Place 18-inch-long piece of plastic food wrap onto counter surface. Pipe each color onto plastic wrap to form wide 12-inch horizontal lines (using all of frosting) that are touching.
• Gently roll frosting lines into one large log using plastic wrap. Snip tip off one end of plastic wrap. Place cut-side down into piping bag fitted with size 1M open-star tip. Pipe rainbow rosettes onto cake beginning with middle of top of cake and working out and down sides so the most vibrant colors are on top of cake.

Steps:

• Preheat oven to 350 degrees F (175 degrees C). Grease and lightly flour four 9-inch cake pans.
• Whisk flour, 1 teaspoon salt, baking powder, and baking soda together in a bowl.
• Beat 1 1/2 cup butter in a stand mixer fitted with the paddle attachment until creamy, about 1 minute. Beat in white sugar on high speed, scraping the bowl with a spatula as needed, until fully incorporated, about 5 minutes. Reduce speed to medium-high; beat in eggs one at a time. Beat in 1 tablespoon vanilla extract.
• Mix flour mixture into the creamed butter mixture in 3 additions, alternating with buttermilk, until batter is smooth and slightly thick.
• Divide batter evenly among 4 bowls. Tint each portion of batter with green, blue, purple, or pink food coloring; stir until smooth. Pour batter into the prepared baking pans.
• Bake in the preheated oven until a toothpick inserted into the center comes out clean, about 25 minutes. Set baking pans on wire racks and let cakes cool completely, about 25 minutes.
• Beat 1 1/2 cup butter and cream cheese in a stand mixer fitted with a clean paddle attachment until creamy, about 2 minutes. Add confectioners' sugar, heavy cream, and 1 tablespoon vanilla extract; beat on low speed for 30 to 60 seconds.
• Increase speed to high and beat frosting until fluffy, about 3 minutes. Scrape down the sides of the bowl as needed. Beat in 1/8 teaspoon salt.
• Cut a 4-inch circle out of the center of 2 cake layers using a round cookie cutter.
• Place one of the intact cake layers on a cake stand or serving plate. Frost the top using an offset spatula or butter knife. Position one of the cake rings on top. Spread frosting over the top and inside the circle. Repeat with second cake ring. Fill the hole with candies and sprinkles. Place remaining cake layer on top. Frost the top and sides of the entire cake.
• Tint any remaining frosting with pink food coloring and pipe decoratively over the cake. Cut cake into slices; spoon any candies that spill out over the slices.