In the bowl of a stand mixer, on medium speed, cream together butter and icing sugar for about 8 minutes with the paddle attachment. Stop halfway to scrape down the sides to ensure it all gets well aerated.

Add vanilla and mix again until combined, about 30 seconds.

In a medium bowl, whisk together the all-purpose flour, cornstarch, and salt. Transfer half to the stand mixer and mix on low to start, working up to medium speed, until mostly combined. Add the remaining flour mixture the same way until no dry streaks of flour remain. Chill the dough for 30 minutes.

Make dough balls with a 1 ½ tablespoon cookie scoop, then roll and place on the prepared baking sheet with about 2 inches between (this should give you 22 cookies).

In a small bowl, combine a small amount of powdered sugar and cornstarch, and dip a fork into the mixture and gently flatten the tops of each cookie. If using, add sprinkles or sanding sugar, then chill for an additional hour.

Preheat oven to 300F and bake cookies for 17 minutes, or just before they start turning golden around the edges, to keep their light color.

Remove and let cool fully on a wire baking rack.