Instructions

• Preheat the oven to 400°F. Grease and flour a large baking sheet (at least 18 x 13 x 1.5 inches) and set it aside.
• In a medium bowl, combine 2 cups all-purpose flour and 2 cups granulated sugar; set aside.
• In a medium saucepan, combine 1/2 cup unsalted butter, 3 1/2 tablespoons unsweetened cocoa powder, 1/2 cup vegetable shortening, and 1 cup water. Heat over medium, stirring, and bring to a boil.
• Immediately pour the hot cocoa mixture over the flour-and-sugar mixture. Using a hand mixer or a stand mixer fitted with the paddle attachment, mix until combined.
• Add 1 teaspoon vanilla extract, 1/2 cup buttermilk, 1 teaspoon baking soda, and 1/2 teaspoon salt to the batter. Mix until combined.
• Add 2 large eggs and mix until the batter is smooth and fully incorporated.
• Pour the batter into the prepared baking sheet. Tap the pan on the counter a few times to release large air bubbles.
• Bake for 20 minutes, or until the center springs back when gently pressed. Remove from the oven and let the cake cool completely in the pan before frosting.
• For the frosting, combine 1/2 cup milk, 1 1/2 cups unsalted butter, and 5 tablespoons unsweetened cocoa powder in the same saucepan. Heat over medium, stirring frequently, until the mixture just reaches a boil. (You can reuse the same saucepan; no need to wash it first.)
• Remove the saucepan from the heat and pour the hot mixture into a medium bowl. Using a hand mixer or a whisk, stir in 3 cups powdered sugar, 1 teaspoon vanilla extract, and 1 dash salt until the frosting is smooth.
• Let the frosting cool and thicken for about 10 minutes, then spread it evenly over the completely cooled cake.
• Immediately sprinkle 1 cup chopped pecans or walnuts over the frosting, if desired.