In a bowl, combine the butter and sugar. Add eggs, one at a time, stirring well after each addition. Stir in vanilla. Combine the flour, cocoa, baking powder and salt. Gradually add to butter mixture; stir until moistened.
Transfer to a greased 13x9x2-inch baking pan. Bake at 350 for 20-25 minutes or until a toothpick inserted near the center comes out clean. Cool on a wire rack.