Cream the butter, sugar and cocoa, Then add eggs 1 at a time mixing well, then add flour and vanilla, mix well, but do not over beat.
Pour Batter into a greased and floured 9X13 pan. Bake in a preheated 325degree oven for 40 min.
While cake is baking, cream butter, throughly,add sugar blend well. Add cocoa, and powdered sugar and mix well on low, till sugar is well mixed in.then on medium till well blended. Add vanilla, and 1 Tbsp milk at a time up to 3 till desired consistancy. Spread on a warm cake and let completely cool before serving.