Preheat oven to 400°F. Grease a 10 x 15-inch jelly roll pan and set aside.
Place butter, shortening, water, and cocoa powder in a saucepan. Bring to a boil; remove from heat.
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.
Add eggs, vanilla extract and buttermilk. Beat on medium speed for 1 more minute.
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. 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.
FOR THE FROSTING
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.
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!
Notes
White Texas Sheet Cake: My mother-in-law often tweaks this recipe to make a vanilla version instead (the grandchildren adore it). It includes almond extract and a buttercream frosting! For the full instructions, see this recipe.
Add Pecans: Sprinkle about ¾ cup of chopped, toasted pecans on the hot frosting.
Decorate the Cake: Add colored sprinkles on top of the hot frosting. This is great for a birthday cake, but you can also tailor the sprinkles for different holidays -- orange and black for Halloween, red and green for Christmas, or pink and red for Valentine's Day.
Buttermilk is an important ingredient in this chocolate cake recipe, so do not substitute with regular milk. The acid in the buttermilk helps to keep the cake moist and tender, and also reacts with the baking soda to help the cake rise.
Sift the confectioners' sugar to avoid lumps in the frosting.