Cream butter, sugar, crushed candy, and vanilla thoroughly using an electric mixer. Gradually blend in flour and cornstarch.
Form dough into 1-inch balls and place on a parchment paper-lined baking sheets. Gently press down on each cookie to flatten, either using fingers or a flat-bottomed drinking glass that has been dipped in sugar to prevent sticking.
Bake for 25 to 30 minutes, or until bottoms begin to brown.
Cool for 5 minutes before removing to a wire rack to cool completely.
For frosting: combine powdered sugar, milk, and vanilla. Mix thoroughly until frosting is smooth. Drizzle cookies with frosting and sprinkle with crushed candy.