Preheat oven to 350 degrees. Line a 15 x 10 x 1 inch baking sheet with parchment paper, spray with no-stick spray. In a saucepan, melt the butter, cocoa powder and water until the butter is melted, stir frequently, set aside to cool slightly. In a large mixing bowl, combine the flour, sugar, baking soda, vanilla, eggs, salt and buttermilk.. Stir to combine. Add the chocolate mixture gradually - stir until smooth. Pour into the prepared baking sheet and bake for 20 minutes. Just prior to the brownies finishing baking - make the frosting. Melt the butter, cocoa and buttermilk in a large saucepan (you will be making the frosting in this pan). When the butter has melted, add the remaining ingredients and stir until smooth. Spread over the warm brownies, sprinkle with chopped nuts, if desired. Allow to cool prior to cutting.