The most extraordinary moist three-layer Yellow Cake with Chocolate Frosting made by preheating oven to 350°F greasing flouring and lining three 8-inch pans whisking 3 cups flour 2 tsp baking powder 1/2 tsp baking soda and 1/2 tsp salt beating 1 cup softened butter medium-high 1 to 2 minutes adding 2 cups sugar beating 2 minutes pale and fluffy adding 4 room temperature eggs and 2 extra yolks one at a time beating thoroughly then vanilla whisking 3/4 cup room temperature buttermilk and 1/4 cup room temperature sour cream on low alternating flour in 3 additions with buttermilk sour cream in 2 additions beginning and ending with flour just combined dividing evenly among pans baking 22 to 26 minutes until toothpick clean cooling 10 minutes then on racks then beating 1 1/2 cups room temperature butter 3 minutes pale and fluffy adding 1 1/4 cups sifted cocoa beating incorporated gradually adding 5 to 6 cups sifted powdered sugar alternating with 1/2 cup cream adding vanilla and salt beating 3 minutes smooth fluffy and fudgy assembling three layers with frosting between crumb coating refrigerating 20 minutes applying final layer. A completely perfect moist buttermilk sour cream three-layer golden egg yolk yellow birthday cake deeply fudgy cocoa frosting recipe.