In medium saucepan,combine butter,water,and cocoa.
Cook and stir till mixture comes to boiling.Remove from heat and set aside.
In a bowl,stir together flour,sugar,soda,and salt.Add buttermilk,eggs,and vanilla;beat smooth.
Stir batter into hot cocoa mixture.By hand,beat till well combined.
Pour into greased and floured 15 1/2x10 1/2 x 1-inch baking pan.
Bake in 400* oven for 20 minutes.
While warm,pour chocolate buttermilk frosting atop.
Cut into bars when cool.Makes 24.