In a microwave, melt butter and chocolate; stir until smooth. Cool slightly. In a large bowl, beat eggs for 2 minutes. Gradually add sugar, beating until thick and pale yellow. Combine flour and baking powder; add to egg mixture. Stir in the melted chocolate mixture, vanilla and chips.
Pour into a greased 8-in. square microwave-safe dish. Cook on high for 3-1/2 to 4 minutes or until a toothpick inserted in the center comes out clean. Place on a wire rack; cool for 10 minutes. Dust with confectioners' sugar.