In a large saucepan, bring butter, cocoa and water just to a boil. Cool. , Meanwhile, preheat oven to 350°. In a large bowl, combine the flour, sugar, baking soda and salt. Add cocoa mixture and buttermilk; mix well. Beat in the eggs, vanilla and, if desired, food coloring. Pour into a greased and floured 15x10x1-in. baking pan. , Bake until set, 20 minutes. For frosting, melt butter with cocoa and buttermilk. Stir in the confectioners' sugar, vanilla and salt. Spread over warm cake. Garnish with nuts if desired.