Preheat the oven to 300°F and prepare your cookie sheets with parchment paper or silicone mats.

In a large mixing bowl, cream together the butter and powdered sugar until the mixture is fluffy and light, which should take around 5 minutes.

Gently add in the all-purpose flour, cornstarch, salt, and vanilla extract. Mix until everything is well combined.

Lightly dust your hands with cornstarch to handle the dough, then roll it into small balls, roughly 2 teaspoons to 1 tablespoon in size.

Arrange the dough balls on the prepared sheets and flatten each one using the tines of a fork that has been dipped in cornstarch to avoid sticking.

Sprinkle a few colorful sprinkles on top of each cookie.

Bake in the preheated oven for 14 to 16 minutes, or until the cookies are set on top, but not browned.