Step 1

Preheat oven to 400 degrees F. Grease a 10 x 15-inch jelly roll pan and set aside.

Step 2

Place butter, shortening, water and cocoa powder in a saucepan. Bring to a boil; remove from heat.

Step 3

In a large bowl, whisk together sugar, flour, baking soda and salt. Pour the hot butter mixture over the flour mixture. Use an electric mixer to beat on medium speed for 1 minute.

Step 4

Add eggs, vanilla extract and buttermilk. Beat on medium speed for 1 more minute.

Step 5

Pour the batter into the prepared pan. Tap on the counter a few times to release air bubbles. Bake for 20 minutes, or until a toothpick inserted in the center comes out clean. Place the cake pan on a wire rack and immediately prepare the frosting.

Step 6

If your cake has a rounded top on it when it comes out of the oven, use your hands, oven mits, or a dish towel to gently press down on the top of the cake to create a flat surface. You need a relatively flat top so that the icing will not run off the sides.

Step 7

Place butter, cocoa powder and milk in a saucepan. Bring to a boil; remove from heat. Add the sifted confectioners’ sugar and the vanilla extract; beat until smooth.

Step 8

Pour the hot frosting over the warm cake. Spread evenly, then set the cake on a wire rack to cool completely. The frosting will firm up as it cools. Slice into squares and serve!