Preheat oven 350ºF
Measure the flour, sugar and baking soda and set aside.
Bring 2 sticks butter, cocoa, and water to boil. Remove from heat; stir in dry ingredients.
Add the eggs, buttermilk, and vanilla. Stir until blended well; pour in 9X13 inch greased and floured pan.
Bake at 350ºF for 30 minutes. Remove from oven.
While cake is baking, bring 1 stick butter, cocoa and buttermilk to boil; add 1 box powdered sugar. Stir only until smooth.
Add 1 tsp. vanilla and pecans. Pour frosting over cake.