Cream butter and brown sugar in large mixing bowl. Beat in egg and vanilla. Stir together flour, baking powder, soda and salt. Blend into creamed mixture. Spread batter in 2-quart utility dish. Swirl melted chocolate over top with knife. Microwave until top springs back when lightly pressed with finger (about 8 1/2 to 9 minutes). Turn dish once or twice during cooking time. Cool. Cut into squares. Bob/Sc