- Preheat oven to 400F. Grease a 15-by-10-inch baking pan.
- To prepare brownies, combine granulated sugar, flour and cocoa; mix well.
- Combine 1 cup water with oil and butter in a medium saucepan. Bring to a boil; add to flour mixture and mix well. Add buttermilk, baking soda, eggs and vanilla. Beat well by hand and pour into pan. Bake 20 to 25 minutes, until a wooden pick inserted in the center comes out almost clean. Cool completely on wire racks.
- To prepare frosting, combine butter, cocoa and buttermilk in a saucepan; bring to a boil. Remove from heat. Gradually beat in confectioners' sugar and vanilla with a mixer at low speed. Spread evenly over cooled brownies.