The most extraordinary moist Yellow Sheet Cake with Chocolate Frosting made by preheating oven to 350°F greasing and lining 9×13 pan 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 minute creamy adding 2 cups sugar beating 2 minutes light and fluffy adding 3 room temperature eggs and 2 extra yolks one at a time beating thoroughly then 1 tbsp vanilla on low alternating flour in 3 additions with 1 cup room temperature buttermilk in 2 additions beginning and ending with flour just combined pouring into pan smoothing baking 30 to 35 minutes until toothpick clean cooling completely minimum 1 hour beating 1 1/2 cups room temperature butter 3 minutes pale and fluffy adding 1 cup sifted Dutch cocoa beating smoothly incorporated gradually adding 5 to 6 cups sifted powdered sugar alternating with 1/2 cup cream adding vanilla and salt beating medium-high 3 more minutes incredibly fluffy and fudgy spreading generously over cooled cake decorating with sprinkles. A completely perfect moist golden egg yolk buttermilk yellow sheet cake Dutch cocoa fudge buttercream recipe.