Line two sheet pans with parchment paper and preheat the oven to 350°F.
In a medium mixing bowl, whisk together the flour and cornstarch.
Beat the softened butter and powdered sugar in a mixer for 6 to 7 minutes, until very pale, light, and fluffy. The mixture should resemble whipped buttercream.
Mix in the vanilla extract. Slowly add the flour mixture and mix just until combined.
Refrigerate the dough for 30 minutes.
Using a #60 cookie scoop or about 1 tablespoon, portion the dough and roll each portion into a smooth ball. Place the dough balls on the prepared sheet pans approximately 1 inch apart.
Gently press each cookie with a fork to flatten it slightly. If the dough sticks, allow it to lift with the fork, then gently peel the dough away from the back of the fork toward the front using your fingers. Add sprinkles before baking, if desired.
Bake for 9 to 11 minutes, until the edges are firm and just beginning to turn lightly golden. The tops should remain pale.
Allow the cookies to cool completely on the sheet pans before serving.