To make Texas sheet cake, combine 2 cups all-purpose flour, 2 cups sugar and 1/4 teaspoon salt. In a saucepan, heat 1 cup butter, 1 cup water and 3 tablespoons unsweetened baking cocoa to boiling. Pour the butter mixture into the flour mixture and stir well. Add 1/2 cup buttermilk, 1 teaspoon baking soda, 2 eggs and 1 teaspoon vanilla. Stir well, pour the mixture onto a baking sheet, and bake at 325 degrees Fahrenheit for 22 to 25 minutes.
Cook the cake in a jelly roll pan, or in a 10- by 15- by 1-inch baking pan. Spay the pan with cooking spray before pouring the batter in it. To make the frosting, combine 1/2 cup butter, 3 tablespoons unsweetened baking cocoa and 6 tablespoons milk.
Bring the mixture to a simmer, remove it from heat, and stir in 2 1/2 cups powdered sugar and 1 teaspoon vanilla. Stir in 1 cup chopped pecans, and pour the warm frosting over the cake. Allow the cake to cool for one hour before serving.
Alternately, add 1 teaspoon instant coffee to the cake batter and the frosting mixture to make a mocha sheet cake. Another option is to add 1 teaspoon cinnamon to the buttermilk mixture before mixing it into the dry ingredients. If desired, omit the pecans from the frosting recipe.