Preheat your oven to 300°F. Line two large baking sheets with parchment paper or silicone baking mats.
In a large mixing bowl, combine the softened butter and sifted powdered sugar. Beat on medium-high speed for a full 5 minutes until the mixture turns pale, fluffy, and cloud-like. Do not rush this step!
Gradually add the all-purpose flour, cornstarch, salt, and vanilla extract to the whipped base. Mix on low speed just until a soft, uniform dough forms.
Scoop the dough into small balls (about 1 tablespoon each) and arrange them on your prepared baking sheets. If the dough sticks, lightly dust your hands with extra cornstarch.
Dip the tines of a kitchen fork in cornstarch and gently press flat onto the top of each ball to create a classic, old-school crisscross shortbread pattern.
Top the center of each pressed cookie with a generous pinch of sprinkles or a piece of candied cherry.
Bake at 300°F for 14 to 16 minutes. Check them at the 8-minute mark if your oven runs hot. The tops should remain pale and stable, with no golden browning.
Remove from the oven and let the cookies cool completely directly on the baking sheet. They are incredibly fragile when warm but will firm up into a melt-in-your-mouth crumb once cooled.