1 box Betty Crocker® SuperMoist® white cake mix
1 1/4 cups buttermilk
1/3 cup vegetable oil
1 tablespoon unsweetened baking cocoa
2 teaspoons royal blue paste food color
1 toothpickful violet paste food color
1 jar (7 oz) marshmallow creme
1 cup butter or margarine, softened
2 1/2 cups powdered sugar
1/8 teaspoon salt
1. Heat oven to 325°F. Grease bottom and sides of three 8-inch round cake pans with shortening and lightly flour, or spray with baking spray with flour.
2. In large bowl, beat all cake ingredients with electric mixer on low speed about 30 seconds or until moistened. Beat on medium speed 2 minutes, scraping bowl occasionally.
3. Divide batter evenly among pans. Bake 23 to 28 minutes or until top springs back when lightly touched in center. Cool 15 minutes. Remove from pans; cool completely.
4. In large microwavable bowl, microwave marshmallow creme uncovered on High 15 to 20 seconds to soften. Add butter. Beat with electric mixer on medium speed until smooth. Beat in powdered sugar and salt until smooth.
5. If necessary, trim rounded tops of two cake layers to flatten before assembling. Place 1 cake layer, top side down, on serving plate; spread with about 1/3 cup frosting. Top with second layer, top side down; spread with about 1/3 cup frosting. Top with untrimmed cake layer, top side up. Frost side and top of cake with remaining frosting.