In a large bowl, beat butter, sugar and cornstarch until blended. Beat in flavoring. Gradually beat in flour., Shape dough into a disk; cover. Refrigerate until firm enough to roll, about 45 minutes., Preheat oven to 325°. On a lightly floured surface, roll dough to 1/4-in. thickness. Cut with a floured 2-3/4-in. leaf-shaped cookie cutter. Place 1 in. apart on parchment-lined baking sheets., Bake until edges are light brown, 20-25 minutes. Remove from pans to wire racks to cool completely., In a small bowl, mix confectioners' sugar and maple syrup until smooth. Spread over cookies. Let stand until set.