In a small bowl, use a hand mixer or stand mixer to cream the butter and confectioners' sugar until light and fluffy, three to four minutes. Beat in the extract.
In another bowl, whisk together the flour and cornstarch. Gradually beat the dry mixture into the creamed mixture until just combined.
Cover the bowl with storage wrap and refrigerate until firm enough to handle, about 30 minutes. Preheat the oven to 350°F.
Shape dough into 1-inch balls. Place the balls 2 inches apart on ungreased baking sheets. Bake until the bottoms are light brown, 9 to 11 minutes. Remove the cookies from the pans and place on wire racks. Cool completely to room temperature.
In a small bowl, use a hand mixer or stand mixer to beat the butter until fluffy. Beat in the milk, extract and, if desired, food coloring. Gradually beat in the confectioners' sugar until smooth.
Spread the frosting over the cookies. Sprinkle with crushed candies.
