How To Make the Ultimate Rainbow Surprise Cake

Name: How To Make the Ultimate Rainbow Surprise Cake
Yield: 18
Ingredients: 8 Egg yolks, large; 1 1/2 tbsp Baking powder; 4 1/2 cups Cake flour; 6 tsp Gel food coloring; 3 cups Granulated sugar; 7 cups Powdered sugar; 1 tsp Salt, fine; 1 2/3 tbsp Vanilla; 1 Cooking spray; 3 sticks Butter, unsalted; 1 lb Butter, unsalted; 2 tbsp Heavy cream or milk; 2 cups Whole milk