5 sticks (3½ cups) unsalted butter, at room temperature, plus more for pans
2 recipes Classic Vanilla Cake batter
24oz. marshmallow crème (1 16-ounce container plus 1 8-ounce container)
2tsp. pure vanilla extract
1lb. confectioners’ sugar
Blue paste food coloring
1 35-ounce bag round rainbow candies (we used Sixlets; see tip below)
Chocolate sprinkles
Directions:
Heat oven to 350°F. Grease bottoms of one 8-inch and one 10- by 2-inch round cake pan and line with parchment or waxed paper.
Pour 6 cups batter into 10-inch pan and 4 cups into 8-inch pan (any remaining batter can be baked into cupcakes if desired). Bake until each cake is golden and toothpick inserted in center comes out clean, 25 to 35 min. Transfer cakes to wire racks; let cool 15 min. Remove cakes from pans and let cool completely on racks.
Meanwhile, prepare marshmallow frosting: Spoon marshmallow crème into large bowl. Using electric mixer on medium speed, beat in softened butter 2 tablespoons at a time, beating well after each addition. Beat in vanilla. On low speed, gradually add sugar and beat until smooth. Increase speed and beat until light and fluffy. Makes about 5 cups frosting.
Fit pastry bag with 1/2-inch round tip. Spoon in 1 cup frosting; set aside for clouds. Transfer 21/2 cups frosting to medium bowl, then tint light blue with food coloring. Keep remaining frosting white for rainbow.
Trim top of 8-inch round cake to make flat. Cut cake crosswise in half to make 2 semicircles. Spread some of white frosting on trimmed side of 1 semicircle. Top with other cake half. Trim base of semicircle cake to make level and ensure that sides are straight. Line baking sheet with waxed paper. Cut thin piece of cardboard into rectangle same size as base of cake (8 inches by 3 inches) and place underneath cake. Transfer cake on cardboard base to prepared baking sheet. Spread enough frosting to fill gap at top and sides and smooth with offset spatula. Refrigerate until firm, about 30 minutes.
Trim top of 10-inch cake to make level. Place cake, trimmed side down, on serving platter. Spread light blue frosting over cake and smooth with spatula. Cut 4 straws to same height as cake. Insert straws, level with cake height, in row down center of cake to help support rainbow cake. Refrigerate while working on rainbow cake.
Spread remaining white frosting over semicircle cake and make smooth. (Make sure cake is level and sides are straight.) Starting on top edge of cake, press single row of yellow candies close together. Repeat going down side of cake with orange, pink, red, green, light blue, and purple candies (there will be a small opening of just frosting). Repeat on other side of cake starting with purple candies, working down with light blue, green, red, pink, and orange and ending with yellow. Fill in sides of arc with row of yellow next to edge of yellow and continue with orange, red, etc. Refrigerate cake at least 30 minutes.
Move chilled cakes from refrigerator. Carefully transfer rainbow cake to top of blue cake on top of straw supports. Using pastry bag with white frosting, pipe clouds on blue cake and on rainbow cake. Using tweezers, attach sprinkles to make cute faces.
