This Best of the Best Texas Sheet Cake is amazingly fudgy and creamy and it’s even better the next day. It comes together quickly and everyone will LOVE it. It's designed to be made in a standard 10" x 15" or an 11" x 15" sheet pan. so don't substitute a different pan size.
Preheat oven to 350 degrees F (175 degrees C). Grease a 10" x 15" or 11" x 15" inch sheet pan with butter then dust lightly with flour.
In a large sauce pan, over low heat, melt 2 sticks butter. When the butter is melted add 1 cup water and whisk in 5 tablespoons cocoa powder. Bring the mixture to a boil then remove from heat. Allow the cocoa mixture to cool slightly.
Meanwhile, in a large bowl, combine 2 cups all purpose flour, 2 cups sugar, 1 teaspoon baking soda and 1/2 teaspoon salt. Stir to mix.
Add the cocoa mixture to the dry mixture, stirring continuously, until blended.
In a small bowl, beat together 2 eggs and 1/2 cup sour cream until combined. Slowly add the egg mixture to the large bowl containing the cocoa and flour mixture.
Pour batter into prepared pan. Bake for 20 minutes. At 20 minutes check and continuing baking until a toothpick inserted into the center comes out clean.
When there is 10 minutes baking time left on the cake, make the frosting. In a large saucepan, over low heat, add 1/4 cup plus 2 tablespoons milk and 1 stick butter. When melted, remove from heat and slowly whisk in ¼ cup plus 1 tablespoon cocoa until combined. Stir in 4 cups confectioners' sugar and 1 ½ teaspoon vanilla, then fold in the nuts, mixing until blended. When the cake comes out of the oven, spread frosting over hot or warm cake.