Mix together sugar and flour and set aside. Bring to a boil the butter, water and cocoa. Pour over flour mixture and stir. Add buttermilk, vanilla, eggs and baking soda. Mix well and pour onto a cookie sheet. Bake 20 minutes at 375 F. Let cool til just warm and frost with Sheet Cake Frosting. Frosting: Melt butter, milk and cocoa together. Remove from heat. Add powdered sugar, vanilla extract and chopped nuts. Beat well and spread on frosting while both cake and frosting are warm.