• Preheat the oven to 400 degrees Fahrenheit. Grease and flour a large baking sheet that is at least 18 x 13 x 1.5 inches and set it aside.
• Combine 2 cups all-purpose flour and 2 cups granulated sugar in a medium bowl.
• Add ½ cup unsalted butter, 3 ½ tablespoons unsweetened cocoa powder, ½ cup vegetable shortening, and 1 cup water to a medium saucepan and bring it to a boil over medium heat.
• Pour the cocoa mixture over the flour mixture and mix with a hand mixer or stand mixer fitted with the paddle attachment until combined. Add 1 teaspoon vanilla extract, ½ cup buttermilk, 1 teaspoon baking soda, and ½ teaspoon salt and mix until combined. Add 2 large eggs and mix until combined.
• Pour the batter into the prepared baking sheet and tap the pan on the counter a few times to pop some of the larger bubbles in the batter. Bake for 20 minutes, or until the center of the cake springs back when gently pressed.
• Allow the cake to cool completely before frosting.

Frosting

• Add ½ cup milk, 1 ½ cup unsalted butter, and 5 tablespoons unsweetened cocoa powderr to the same saucepan used for the cake (no need to clean it if you don’t want to) and heat over medium heat until just boiling. Stir frequently to avoid burning!
• Remove the pan from the heat and pour the cocoa mixture into a medium bowl. Use a hand mixer or a whisk to stir in 3 cups powdered sugar, 1 teaspoon vanilla extract, and 1 dash salt until combined and smooth.
• Allow the frosting to cool and thicken for 10 minutes or so before spreading it evenly over the cooled cake. Top the frosting immediately with 1 cup chopped pecans or walnuts if desired.