Place the softened salted butter and sifted powdered sugar in a large mixing bowl. Beat them together for 3–4 minutes, allowing enough mixing time to develop the light texture required for these cookies.
Add the vanilla extract and continue beating briefly until it is distributed evenly throughout the butter mixture.
Add the all-purpose flour and cornstarch to the bowl.
Mix only until the dry ingredients have been fully incorporated and a cohesive dough forms. Avoid excessive mixing, as working the dough too long may produce a firmer cookie texture.
Divide the prepared dough into 36 evenly sized portions and roll each portion into a ball.
Arrange the dough balls on a lined baking sheet. Alternatively, place them on a large plate that has been lightly dusted with flour to reduce sticking.
Dip the tines of a fork into either cornstarch or flour, then gently press the top of each dough ball. Decorate with the festive sprinkles as desired.
Refrigerate the shaped cookie dough for approximately 2 hours, or until the portions feel very firm.
For a faster chilling method, place the dough balls in the freezer until sufficiently firm. Proper chilling helps the cookies retain their shape while baking.
While the dough is chilling, preheat the oven to 300°F.
Transfer the firm dough balls to a baking sheet, leaving even spacing between them. Approximately 12 cookies may be baked on one tray at a time.
Bake for 19–20 minutes, or until the tops of the cookies appear set.
Leave the freshly baked cookies on the baking sheet until they have cooled enough to handle. Once completely cooled, they are ready to serve.