Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 9x13 inch pan. In a saucepan, combine 1/2 cup butter, oil, 4 tablespoons cocoa, and 1 cup water. Boil for 1 minute.
In a large bowl, mix flour and white sugar. Beat in chocolate mixture. Beat in eggs, buttermilk and 1 teaspoon vanilla. Pour batter into prepared pan.
Bake in the preheated oven for 25 minutes, or until a toothpick inserted into the center of the cake comes out clean. Allow to cool.
To make the Frosting: In saucepan, combine 1/2 cup butter, 6 tablespoons cream, and 4 tablespoons cocoa. Boil for 1 minute. Place confectioners' sugar in a large bowl. Pour in chocolate mixture and beat until desired consistency is achieved. Beat in vanilla.
