HEAT oven to 350°F. Beat butter and super-fine sugar in large bowl with mixer on high speed until light and fluffy. Combine flour and cornstarch. Gradually add to butter mixture, beating well after each addition. Stir in chocolate.

DROP by heaping tablespoonfuls, 1 inch apart, on baking sheets.

BAKE 20 to 25 min. or until lightly browned. Cool 5 min. on baking sheets. Transfer to wire racks; cool completely. Sprinkle with icing sugar.