In medium saucepan, melt 2 sticks (1 C) butter, 4 T cocoa, and water. Bring to boil then remove from heat, mixing well.
In large mixing bowl, add flour and sugar. Stir to mix well.
Pour hot mixture over flour-sugar mixture. Mix well. Add buttermilk, baking soda, eggs, cinnamon, and 1 tsp vanilla. Mix well.
Pour into buttered and floured 9×13 pan. Bake at 400 degrees for 20-25 minutes.
Make icing while cake bakes: Reuse medium saucepan–melt 1 stick butter and add 4 T cocoa, 6 T milk (or 3 T milk and 3 T, 1/2 & 1/2), 1 box powdered sugar, and 1 tsp vanilla. Mix well before adding pecans.
Wait if you want, but you can ice this cake hot out of the oven. Icing will thicken as it cools. Let cool before serving. Delicious by itself or a la mode.