White Cream Cake:

Let 24 tbsp (3 sticks) of unsalted butter, 9 large egg whites, and 1 3/4 cups of buttermilk come completely to room temperature.

Preheat your oven to 350 F and prep two 9" round cake pans. Butter the sides and bottoms of the pans, and line the bottom with parchment paper. Prep 4 additional rounds of parchment paper (a total of six 9" rounds of parchment paper will be needed).

In a medium-sized mixing bowl, add in the 1 3/4 cups of buttermilk, 3/4 cup of vegetable oil, and 1 tbsp of clear vanilla extract. Mix to combine and set this aside.

Take a large mixing bowl and a sieve, add in 4 1/2 cups of cake flour, 1 1/2 tbsp of baking powder, 3/4 tsp baking soda, and 1 tsp of salt. Sift them in, and whisk them together to combine and set aside.

In the bowl of your stand mixer, add in the 24 tbsp of room temperature butter. With a paddle attachment, mix on medium-high speed until the butter is smooth. While continuing to mix on medium-high speed, slowly add in 2 2/3 cups of granulated sugar. Once all the sugar has been added, continue whipping on medium-high speed for 5 minutes, until the mixture looks light and fluffy.

Turn the speed down to medium-low, and slowly add in the 9 room temperature egg whites, one-third at a time. It's very important that the egg whites are at room temperature, and wait for the added egg white to get fully incorporated before adding more.

When the egg whites are fully incorporated, stop the mixer and scrape down the bowl. Then continue to mix on medium-low, add in one-third of the dry ingredients, and mix until fully incorporated. Then add in one-third of the wet ingredients and mix until fully incorporated. Continue alternating between dry and wet ingredients (one-third at a time), till you've added till everything has been added. Mix until just combined. Scrape down the paddle attachment and mixing bowl as needed.

Divide this batter into 6 different bowls. Mix the food coloring into each bowl of the 6 bowls with the following amounts of coloring: 1/2 tsp AmeriColor Regal Purple, 1/2 tsp AmeriColor Gel Royal Blue, 1/2 tsp Americolor Leaf Green, 1/2 tsp AmeriColor Gel Electric Yellow, 1/2 tsp AmeriColor Gel Orange, 1/2 tsp AmeriColor Gel Super Red. Mix these colors in so that each bowl of colored batter is a nice uniform color.

Since we only have 2 round pans, we'll be baking the cakes in 3 batches. Pour the first 2 colors into the 2 prepared pans, spreading the batter evenly into the pan with a small spoon.

Bake in your preheated oven at 350 F for 15 minutes, or until a toothpick inserted in the center comes out clean and the sides of the cake start to pull away from the sides of the pan.

Let them cool in the pans for 5 minutes on a metal rack. Then carefully invert the cakes onto metal racks, lift off the pans, peel off the parchment paper the cakes were baked with, and let them finish cooling to room temperature on the metal rack.

Repeat these steps with the other two batches (remaining 4 colors). Wipe off the pans with a paper towel (no need to wash them). Prep them again: butter the sides and bottoms of the pans, and line the bottom with parchment paper. Pour the batter in, and repeat steps 10 and 11.

Chocolate Ganache Frosting / Filling:

Take 1 lb. of very high-quality bittersweet (or semisweet) chocolate and grate it into a large bowl.

Take a medium-sized saucepan, add 1 2/3 cups of heavy cream, 1/4 cup of granulated sugar, 1/4 cup of corn syrup, and bring this to a gentle simmer on low-medium heat while mixing till the sugar dissolves. As soon as it starts to bubble at the sides, remove it from heat.

(NOTE: the corn syrup will give the frosting a shiny finish. If corn syrup is not available, you can substitute it with 1/4 cup of additional granulated sugar.)

Add in the 1 lb. of grated bittersweet chocolate and stir to and melt it in. Add 6 tbsp of unsalted butter that's been cut into pieces, 2 tsp of vanilla extract, and 1/4 tsp of kosher salt, and stir this into the mixture till it's smooth.

Place the saucepan over a big bowl of ice water, and continue stirring till it cools and becomes a nice spreadable consistency. This should take about 3 to 5 minutes of stirring in the ice bath. It should not be too cool that it's stiff and hard to work with, but not too warm that it's runny. If it gets too cold and stiff, just let it sit at room temperature to warm up till it becomes spreadable again.

Assembling the Cake:

Take a cake board that's larger than 9" (we use a 10" cake board), place it on a revolving cake stand. Place the purple layer on it.

Frost a thin layer of chocolate frosting, about 1/8" thick layer of frosting or about 1/3 cup worth of frosting, making it as smooth and even as possible. Place the blue layer on top of it.

Repeat the process of adding a very thin layer of chocolate frosting (1/8" thick layer of frosting or about 1/3 cup worth of frosting), alternating with layers of cake. After purple and blue, add the layers of cake in the following order: green, yellow, orange, and red on top.

Use the remaining chocolate frosting to cover the sides and top of the cake. I use a bench scraper to smoothen out the sides and an offset spatula to smoothen out the top as much as possible. This chocolate ganache is rich, and a thin layer of frosting on the outside is enough to result in a moist and flavorful cake. Clean off any excess frosting from the base.

For additional decoration, I like to press rainbow sprinkles around the bottom edge of the sides of the cake, going about 1/3 of the way up the side of the cake. To do this, prepare short strips of parchment paper that are about 2 inches wide by 6 inches long. Spray just a little bit of vegetable oil spray onto the strips. Then add rainbow sprinkles on top of the oiled surface. One strip at a time, pick up the strip of parchment paper, and quickly press the strip onto the chocolate frosting on the sides of the cake. Gently flick the parchment paper so the sprinkles stick to the chocolate, then carefully peel away the parchment paper. Do this around the entire circumference of the cake. Then clean off any excess sprinkles from the base of the cake.

To finish it off, we like to sprinkle just a little bit of rainbow frosting on top of the cake as well to add a pop of color. If you're serving the cake the same day or the following day, keep the cake at room temperature under a cake dome (or loosely with plastic wrap / foil). This chocolate ganache is best fresh and unrefrigerated. For storage longer than 2 days, keep your cake in the fridge till you're ready to serve it. But we recommend taking it out of the fridge about 3 hours before serving to allow it to come to room temperature and let the chocolate ganache soften. Enjoy!