Peanut Butter-Fudge Cake

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