In a large bowl, cream butter and sugar. Add 1 egg at a time, beating well after each addition. Beat in vanilla. Combine the flour, cocoa, baking powder and salt; gradually add to creamed mixture and mix well.
Spread into a greased 13×9-in. baking pan. Bake at 350° for 25-30 minutes or until a toothpick inserted in the center comes out clean. Cool on a wire rack.
Cut into 32 bars. Use frosting to decorate like dominoes.