Preheat the oven to 350 degrees. Spray 2 8-inch round cake pans with cooking spray and set pans aside.

Combine the cake mix, pudding mix, vegetable oil, sour cream, milk, and egg whites in a large mixing bowl. Beat on medium speed for 2 minutes, or until well combined. Pour half of the batter into a second mixing bowl and mix blue food coloring into one of the bowls to achieve your desired shade of blue.

Drop a large spoonful (about 2 tablespoons) of white batter into one of the prepared pans. Top that with a large spoonful of blue batter, and continue layering the batter until you’ve used half of each color. Jiggle the pan occasionally to even the batter out. Repeat with the second cake pan and the remaining batter.

Bake cake layers for 30 to 34 minutes, until a cake tester inserted into the center of the cakes comes out clean. Cool in pans for 20 minutes, then carefully invert onto a wire rack to cool completely.

Place the butter in the bowl of a stand mixer fitted with the paddle attachment and beat until very smooth. Add 4 1/2 cups of the powdered sugar, 3 tablespoons of the milk, and the vanilla, cocoa powder, and salt. Mix on low until well combined then add food coloring to achieve desired color. Turn mixer to medium and mix until food coloring is fully incorporated. Add additional powdered sugar if frosting is too thin and additional milk or cream if it is too thick.

If your cake layers have domed, use a long serrated knife to trim them. Place one cake layer on a cake stand or cardboard cake circle and spread or pipe frosting in an even layer over it. Top with the second cake layer and frost with the remaining frosting. Decorate as desired, then serve.