Using a stand mixer or a hand mixer, combine softened butter, flour, cornstarch, and sugar. Beat the batter for at least 10 minutes, or until you get a very air, fluffy consistency.
Using a cookie scoop, portion out the cookie dough, placing the pieces on a lined baking sheet as you go. If you wish, add sprinkles to the top of each cookie.
Bake the cookies at 300°F for about 20-25 minutes, or until the bottoms turn golden brown. Let the cookies rest for about 5 minutes before serving. Enjoy!