In a bowl, combine the first four ingredients; set aside. In a small saucepan, bring butter, cocoa and water to a boil. Immediately remove from the heat. Add to dry ingredients and mix well. , In a small bowl, beat eggs. Add buttermilk and vanilla; mix well. Stir into cocoa mixture. Pour into a greased 15x10-x1-in. baking pan. Bake at 375° for 20-22 minutes or until a toothpick inserted in the center comes out clean., Meanwhile, in a saucepan, bring the butter, cocoa and milk to a boil, stirring constantly. Remove from the heat; add sugar and vanilla. Mix well. Spread over hot cake; immediately sprinkle with nuts. Cool completely on a wire rack.