3 ½cups(397g)powdered sugar, sifted to avoid lumps
Chopped, toasted pecans (optional)
Instructions
Preheat the oven to 350 degrees F. Lightly grease a half sheet pan (a rimmed baking sheet measuring 11X17-inches). Set aside.
For the cake, in a large bowl, whisk together the granulated sugar, flour, baking soda and salt.
In a medium saucepan, add the butter and water. Sift in the cocoa to avoid lumps. Bring the mixture to a boil. Pour over the dry ingredients and stir until combined.
Add the buttermilk, eggs and vanilla and mix until well-combined.
Spread the batter evenly in the prepared pan. Bake for 20-25 minutes until the cake is evenly baked and springs back lightly to the touch (watch closely; it is a thin cake and bakes quickly).
While the cake bakes, make the frosting.In a medium saucepan, combine the butter, cocoa, milk (or buttermilk), vanilla and salt. Bring the mixture to a simmer over medium heat, stirring constantly.
Immediately remove from the heat and add the sifted powdered sugar a cup at a time, whisking in between additions and mixing until the frosting is smooth. Keep the frosting warm if the cake is not finished baking.
Pour the warm frosting over the warm cake and spread evenly. Top with chopped pecans, if desired. Let the cake cool to room temperature. Serve at room temp or chilled.
Notes
Cocoa Powder: Natural, unsweetened cocoa powder (i.e. Hershey's) works great for both the cake and the frosting. As a slight variation, I'll often sub in 1 tablespoon Dutch-process cocoa in both the cake and the frosting for a deeper, richer chocolate flavor.Pan Size: while a half sheet pan (11X17-inches) is commonly used for Texas sheet cake, you can also bake the cake in a jelly roll pan (15X10-inches) or a 9X13-inch cake pan for thicker cake and frosting layers (increase baking time as needed). If using a jelly roll pan, make sure the sides are high enough so the batter doesn't overflow.