Food & Cooking Recipes Dessert & Treats Recipes Cake Recipes Rainbow Cake 3.7 (805) 82 Reviews By Martha Stewart Test Kitchen Martha Stewart Test Kitchen The recipes developed by our test kitchen team have undergone a rigorous process of development and testing, ensuring that every element is optimal, from ingredient amounts to method and cooking time. This process includes triple-testing recipes to ensure they meet our high standards. The many stellar cooks and food editors who have been part of our team include Sarah Carey, Lucinda Scala Quinn, Jennifer Aaronson, Shira Bocar, Anna Kovel, Greg Lofts, Riley Wofford, Lauren Tyrell, and Lindsay Leopold. Editorial Guidelines Updated on January 16, 2019 Print Rate It Share Share Tweet Pin Email Yield: 1 9-inch-round 6-layer cake This colorful layer cake from "Whisk Kid" blogger Kaitlin Flannery is ideal for a child's birthday party, or any time you want to make a big impression. Ingredients Vegetable shortening 3 cups all-purpose flour 4 teaspoons baking powder ½ teaspoon salt 2 sticks (1 cup) unsalted butter, room temperature 2 ⅓ cups sugar 5 large egg whites, room temperature 2 teaspoons pure vanilla extract 1 ½ cups milk, room temperature Red, orange, yellow, green, blue, and purple gel food coloring Lemony Swiss Meringue Buttercream Frosting Directions Preheat oven to 350 degrees. Brush six 9-inch-round cake pans (or as many 9-inch cake pans as you have, reusing them as necessary) with shortening. Line bottom of each cake pan with parchment paper; brush again and set aside. In a large bowl, whisk together flour, baking powder and salt; set aside. In the bowl of an electric mixer fitted with the paddle attachment, cream together butter and sugar. Slowly add egg whites and mix until well combined. Add vanilla and mix until fully incorporated. Add flour mixture and milk in two alternating additions, beginning with the flour and ending with the milk. Mix until well combined. Divide batter evenly between six medium bowls. Add enough of each color of food coloring to each bowl, whisking, until desired shade is reached. Transfer each color to an individual cake pan. Transfer to oven and bake until a cake tester inserted into the center of each cake comes out clean, about 15 minutes (working in batches if necessary). Remove cakes from oven and transfer to a wire rack; let cool for 10 minutes. Invert cakes onto a wire rack; re-invert and let cool completely. Using a serrated knife, trim tops of cakes to make level. Place four strips of parchment paper around perimeter of a serving plate or lazy Susan. Place the purple layer on the cake plate. Spread a scant 1 cup buttercream filling over the first layer with a small offset spatula so it extends just beyond edges. Repeat process with blue, green, yellow, and orange layers. Place the remaining red layer on top, bottom-side up. Gently sweep away any loose crumbs with a pastry brush. Using an offset spatula, cover the top and sides with a thin layer of frosting (also use any of the excess frosting visible between the layers). Refrigerate until set, about 30 minutes. Using an offset spatula, cover cake again with remaining frosting. Rate it Print