In a mixing bowl, cream peanut butter, butter and sugar. Add eggs; mix well. Combine flour, baking powder and salt; add alternately with milk to creamed mixture. Mix well. Pour into a greased 9-inch square baking pan. Bake at 350° for 30-35 minutes or until a toothpick inserted near the center comes out clean. Cool on a wire rack. For frosting, cream peanut butter and butter. Add sugar, milk and vanilla; mix until smooth. Frost cake.