Preheat oven to 300°F. Line two baking sheets with parchment paper.
Beat softened butter on high until very pale and fluffy, about 3 minutes.
Add confectioners' sugar and vanilla. Beat until light and creamy, 1 to 2 minutes.
Whisk flour, cornstarch, and salt together in a bowl.
Mix dry ingredients into the butter mixture in two additions on low just until combined.
Whip the dough for 30 seconds to aerate. It should be soft and pipeable.
Scoop or pipe 1-tablespoon mounds onto the sheets, spacing about 2 inches apart.
Top with sprinkles, pressing lightly so they adhere.
Bake 14 to 16 minutes until set and pale with no browning on edges.
Cool 5 minutes on the sheet, then move cookies to a rack to finish cooling.