Combine flour, white sugar, and baking soda in a large mixing bowl; set aside.
Melt 1 cup butter or margarine in a heavy saucepan; stir in 1/2 cup cocoa. Stir in buttermilk, and eggs until well blended. Cook over medium heat, stirring constantly, until mixture boils. Remove from heat. Mix into flour mixture, stirring until smooth. Stir in 1 teaspoon vanilla. Pour batter into a greased and floured 13 x 9 inch baking pan.
Bake at 350 degrees F (175 degrees C) for 20 to 25 minutes, or until an inserted wooden pick comes out clean. Cool 10 minutes on a wire rack. Carefully spread peanut butter over warm cake. Cool completely.
To Make Frosting: Combine 1/2 cup butter or margarine, 1/4 cup cocoa, and buttermilk in a small sauce pan. Bring to a boil over medium heat, stirring constantly. Pour over confectioners' sugar, stirring until smooth. Stir in 1 teaspoon vanilla. Spread chocolate frosting over peanut butter on cake. Cut into squares.
