Heat oven to 325°F. Line two baking sheets with parchment.
Beat butter and powdered sugar until pale and creamy, about 2–3 minutes.
Mix in vanilla and salt just until combined.
Whisk flour and cornstarch together in a separate bowl.
Add dry ingredients to the butter mixture. Mix on low until a soft dough forms.
Chill the dough for 15 minutes to firm slightly for easier shaping.
Scoop 1-tbsp portions, roll into balls, and set 2 inches apart on sheets.
Gently flatten each ball with a fork or the bottom of a glass.
Bake 12–14 minutes, until tops look dry and edges are just set without browning.
Cool 5 minutes on the pan, transfer to a rack, then dust with extra powdered sugar.