Mix butter, cocoa powder, and water in saucepan. Bring to a rolling boil; pour over dry ingredients and stir with a wooden spoon or wire whip till blended.
Add buttermilk, vanilla, eggs and soda. Mix well and pour into a greased 11" x 16" x 1/2" jellyroll pan.
Bake 20 minutes at 400° F.
Icing:
1 stick butter
4 tbsp cocoa powder
6 tbsp milk
1 box confectioners' sugar
1 tsp vanilla extract
1 cup chopped walnuts
Begin icing 5 minutes before cake is done.
Melt butter, cocoa, and milk together in saucepan and bring to a boil.
Remove from heat and add confectioners' sugar, vanilla extract, and chopped nuts. Beat well with spoon.