Preheat oven to 350°F and place racks in upper and lower thirds. Butter, flour and line bottom of cake pans with parchment paper. You will need a minimum of 3, 6-inch cake pans to make this recipe. If using three cake pans, divide the ingredients in half and make the recipe twice.
Preparing the batter: To the bowl of a stand mixer fitted with a whisk attachment add egg whites. Beat whites until foamy. Gradually add 1 cup sugar to the whites, beating until soft peaks. Scrape whites into a clean mixing bowl and set aside.
Wipe bowl clean and set on stand mixer fitted with paddle attachment, add flour, remaining 1 1/2 cups sugar, baking powder, and salt. While on medium-low speed add 8 ounces lukewarm water into dry ingredients; increase speed to medium and add in oil, egg yolk, and vanilla. Beat until mixture is smooth and no streaks of flour remain, scrape sides of bowl with silicone spatula as needed. Fold whites into batter in 3 additions.
Divide batter evenly between six clean bowls, each portion will weigh 11 ounces. Add a scant 1/4 teaspoon gel food coloring to each bowl. Adjust coloring as needed, folding coloring through as to not deflate your batter. Gently pour each colored batter into a prepared cake pan.
Transfer cakes to preheated oven, bake until tester inserted into center comes out clean, 15 to 20 minutes. Cool cakes in pans on wire racks for 10 minutes. Turn cakes out of the pans onto the racks and cool completely.
Making the Buttercream Frosting Click Here
Cutting the Cake into Layers: Set cake on cardboard cake round same circumference as cake, then on turntable. First, divide cake, by eye, into 2 layers. Alternately, using a ruler and toothpicks, insert toothpicks around circumference of cake. Each layer will be ¾-1 inch thick. Insert knife into side of cake and, holding knife steady and level, slowly rotate turntable and move knife back and forth in gently swaying motion toward center of cake. Remove top layer and set aside. Repeat as necessary. Brush any loose crumbs off layers and turntable.
Assembling Rainbow Layers (Red, Orange, Yellow, Green, Blue, Violet): If using a syrup, evenly brush or mist over cut surface of bottom cake layer. We began with red. Apply a thin layer of Buttercream Frosting on cake and spread into a uniform layer no thicker than 1/2-inch. Top with second layer of cake and repeat process with additional layers. Straighten layers as necessary until you have all six colors layered.
Crumb Coat: Apply a generous portion of frosting to top of cake. Using a long offset spatula, spread frosting smoothly and evenly to sides of cake, then use overhand to spread frosting around sides of the cake. Hold the spatula vertically at a 90-degree angle to turntable or plate and slowly turn the cake. Remove excess frosting from cake and fill any holes. Set crumb coat by transferring cake to refrigerator or freezer until crumb coat is firm to the touch.
Finishing the cake: Follow crumb coat instructions above, but with a larger quantity of frosting. Finish can be smooth, or using the back of a spoon, swooshes or spikes can be designed in the frosting. A piping bag fitted with a decorative tip can be used to pipe decorations and a decorative border around the cake.
