Preheat oven to 300 degrees F. Line a baking sheet with parchment or a silicone baking sheet. Set aside.
Sift cornstarch, icing sugar and flour together.
Blend in butter with a spoon, mixing until a soft, smooth dough forms. If dough is too soft to form loose balls, add a little bit more flour.
With an electric beater, whip dough on high for about 1 minute.
Shape dough into small 1- inch balls.
Place about 1 1/2 inches apart on an ungreased cookie sheet; flatten slightly with a lightly floured fork.
If adding a topping, add it now, pressing it down slightly, before putting cookies into the oven.
Bake for 20 minutes, or until edges are slightly browned.
Let cool on a rack.