PREHEAT oven to 300°F. Beat butter and sugar in large bowl with electric mixer on high speed 3 min. or until light and fluffy. Combine flour and cornstarch. Gradually add to butter mixture, beating until well blended. Stir in chocolate.
SHAPE dough into ball, then divide into 3 equal pieces. Flatten 1 piece into 4-inch round on baking sheet. Score into 8 wedges with sharp knife. Repeat with remaining dough.
BAKE 23 to 25 min. Cool completely on wire racks. Cut each round into 8 wedges to serve. Store in airtight container at room temperature.