Combine the butter, shortening, water and cocoa in a microwave safe dish and microwave on HIGH for 2 minutes. Pour mixture over flour and sugar. Stir to blend. Add buttermilk**, eggs, baking soda and vanilla. Mix well.
Pour into well-buttered 17 1/2 x 11 inch jelly roll pan. Bake at 400° for 20 minutes or until brownies test done. While brownies bake prepare frosting.
For the frosting combine the butter, cocoa and milk in a microwave safe dish and microwave at HIGH for 2 minutes stirring once. Add powdered sugar and vanilla. Stir to blend. Pour warm frosting over hot brownies. Let cool then cut into bars.
**If you don't have buttermilk use 1/2 cup of milk plus 2 teaspoons of vinegar or lemon juice to sour the milk. Stir and let set for a couple of minutes.