In a saucepan, bring butter, cocoa and water to a boil. Cool. Meanwhile, in a large mixing bowl, combine sugar, flour and salt. Pour cocoa mixture over dry ingredients; mix well. Combine buttermilk and baking soda; add to batter along with eggs, vanilla, and food coloring if desired. Mix until well combined. Pour into a greased 15-in. x 10-in. x 1-in. greased and floured baking pan. Bake at 350 degrees F for 20 minutes. For frosting, melt butter, cocoa and buttermilk in a saucepan. Stir in sugar, vanilla and salt. Spread over warm cake. Top with nuts if desired.