In a small bowl, cream butter and confectioners' sugar until light and fluffy, 3-4 minutes. Beat in extract.
In another bowl, whisk flour and cornstarch; gradually beat into creamed mixture.
Refrigerate, covered, 30 minutes or until firm enough to handle.
Preheat oven to 350°. Bake 9-11 minutes or until bottoms are light brown. Remove from pans to wire racks to cool completely.
Shape dough into 1-in. balls; place 2 in. apart on ungreased baking sheets.
Bake 9-11 minutes or until bottoms are light brown. Remove from pans to wire racks to cool completely.
In a small bowl, beat butter until creamy. Beat in milk, extract and, if desired, food coloring. Gradually beat in confectioners' sugar until smooth. Store in an airtight container.
Spread over cookies; sprinkle with crushed candies.
Store in an airtight container.