Preheat oven to 300°F. Line cookie trays with parchment paper and set aside.
Scrape down the sides of the bowl. Add the sifted Confectioner's sugar. If using vanilla, add now. Beat the mixture for 3 more minutes.
Add the flour and cornstarch mixture into the butter. Whip until combined, about 1 to 2 minutes. Remove the bowl from the stand mixer.