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 3 minutes. 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, beating on low 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 and let 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, then roll in granulated sugar before rolling in confectioners’ sugar. Place on baking sheets.

Bake for 12–13 minutes or until edges are set and centers look soft.

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

Cookies stay fresh covered at room temperature for up to 1 week.