In a microwave-safe bowl, melt butter and chocolate; stir until smooth. Stir in brown sugar until dissolved. Cool to lukewarm. Stir in egg and vanilla. Combine the flour, baking powder and salt; stir into chocolate mixture until blended. Stir in chips.
Spread into a greased 9-in. square baking pan. Bake at 350° for 22-27 minutes or until a toothpick comes out with moist crumbs. Cool on a wire rack.