In a saucepan, bring butter, cocoa and water just to a boil. Cool. Meanwhile, in a large bowl, combine the sugar, flour and baking soda and salt. Pour over dry ingredients; mix well. , Add buttermilk, eggs, vanilla, and food coloring if desired. Mix until well combined. Pour into a greased 15x10x1-in. baking pan. Bake at 350° for 20 minutes. , For frosting, melt butter, cocoa and buttermilk in a saucepan. Stir in the sugar, vanilla and salt. Spread over warm cake. Top with nuts if desired.