Preheat oven to 350°. Combine flour, sugar and baking soda in a large mixing bowl; set aside.
Melt margarine in a heavy saucepan; stir in cocoa. Add water, buttermilk and eggs, stirring well.
Cook over medium heat, stirring constantly until mixture boils. Add to flour mixture; stir until smooth. Stir in vanilla extract. Pour batter into a greased, floured 13-by-9-inch baking pan.
Bake for 20 to 25 minutes or until a toothpick inserted in center comes out clean. Cool for 10 minutes on a wire rack. Spread peanut butter on warm cake. Cool completely for 30 minutes. Then spread Chocolate Frosting on top; cut into squares.