FOR THE CAKE: Place 2 sticks of butter, 4 tablespoons cocoa, and 1 cup water over medium heat and bring to a boil.
Pour over flour and sugar mixture. Mix well. Add eggs, buttermilk (or sour cream), baking soda, salt, and vanilla. Mix well.
Pour into a greased and floured cookie sheet pan (15½ x 10½ inches). My pan was slightly larger; about 18 x 12 inches. Cookie sheet must be a deep cookie sheet pan.
Bake at 350 degrees for 20 minutes, or until a toothpick comes out clean from the center of the cake. Ice while cake is hot.
FOR THE FROSTING: Five minutes before the cake is finished, make frosting. Mix together butter, cocoa and milk. Cook over medium heat and bring to a boil.
Remove from stove and add powdered sugar and vanilla. Mix well. Pour over cake while hot.
Recipe by Dallas Duo Bakes at http://dallasduobakes.com/2013/09/leahs-texas-chocolate-sheet-cake.html