Whisk the flour, cornstarch, baking soda, and salt together in a medium bowl. Set aside.

In a large bowl, beat the butter and granulated sugar together on medium-high speed until creamed, about 1 minute. Add the egg, lemon juice, lemon zest, and vanilla extract and beat on high speed until combined, about 1 minute. Scrape down the sides and add the flour mixture, then beat on low speed until combined.

Cover dough tightly and chill in the refrigerator for at least 3 hours and up to 3 days.

Remove cookie dough from the refrigerator. Let it sit at room temperature for about 10 minutes if chilled longer than 3 hours.

Preheat oven to 350°F (177°C) and line baking sheets with parchment paper.

Scoop and roll balls of dough, about 1 tablespoon each, into balls. Optionally, roll in granulated sugar before rolling in confectioners’ sugar. Place 3 inches apart on the baking sheets.

Bake the cookies for 12–13 minutes or until the edges appear set and the centers look soft.

Cool cookies for 5 minutes on the baking sheet, then transfer to a cooling rack to cool completely.