For cake: Place oven rake in middle position. preheat oven to 350 degrees. Grease and flour a 9 x 13 inch sheet pan. Combine flour, sugar, baking soda and salt in a large bowl. Whisk well.

Combine butter, cocoa and 1 1\2 cups water in a medium sauce pan over medium-low heat. Whisk frequently until butter has melted and mixture is smooth, glossy and bubbling around the edges. Remove from heat.

Fold cocoa mixture into dry ingredients until combined. Stir in buttermilk, eggs and vanilla. pour batter into greased and floured pan. Bake for 25-30 minutes. Remove from oven and cool completely on a oven rack.

For frosting: Combine butter, cocoa and buttermilk in a small saucepan over medium low heat. Whisk until butter has melted and mixture is smooth and bubbling around edges. Remove from heat and transfer to a standing mixer bowl

Mix on low speed, add vanilla, salt, sugar and nuts if using. Pour warm frosting over top of cake and smooth with a spatula. Allow frosting to become firm before slicing.