Beware this cake is very sweet, but great with a glass of cold milk or a cup of strong coffee. You can also make this recipe using 2 13-by-9-inch pans. Adjust the cooking time down to 15 to 20 minutes.
Recipe Source: American Profile
Preheat oven to 400°F Grease a 15 x 10-inch baking pan aka Jelly roll 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 22 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.