Combine first 3 ingredients in a large mixing bowl; set aside.
Melt butter in a heavy saucepan; stir in cocoa. Add water, buttermilk, and eggs, stirring well.
Cook over medium heat, stirring constantly, until mixture boils. Remove from heat; add to flour mixture, stirring until smooth. Stir in vanilla. Pour batter into a greased and floured 13- x 9-inch baking pan.
Bake at 350° for 20 to 25 minutes or until a wooden pick inserted in center comes out clean. Cool 10 minutes on a wire rack. Carefully spread peanut butter over warm cake. Cool completely.
Spread Chocolate Frosting over peanut butter; cut into squares.