In medium microwavable bowl, microwave butter and bittersweet chocolate chips uncovered on High 1 to 2 minutes, stirring once, until chips are softened and can be stirred smooth. Stir in sugar, vanilla and peppermint extract with whisk. Add eggs, one at a time, beating well with whisk after each addition. Stir in flour, baking powder and salt until combined. Pour batter into pan.