Whisk the flour, cornstarch, baking soda, and salt together in a medium bowl. Set aside.
In a large bowl using a hand mixer or a stand mixer fitted with a paddle attachment, 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. The mixture will appear curdled, which is okay. Scrape down the sides and bottom of the bowl as needed. Add the flour mixture and beat on low speed until combined. The dough will be thick, creamy, and sticky.
Cover the dough tightly and chill in the refrigerator for at least 3 hours and up to 3 days.
Remove the cookie dough from the refrigerator. If the dough chilled longer than 3 hours, let it sit at room temperature for about 10 minutes to make it easier to scoop and roll.
Preheat the oven to 350°F (177°C). Line baking sheets with parchment paper or silicone baking mats. Set aside.
Scoop and roll balls of dough, about 1 tablespoon each (20g), into balls. Roll the balls in granulated sugar first (optional) and then very generously 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 still look soft. If the cookies aren’t spreading by minute 9, remove them from the oven and lightly bang the baking sheet on the counter 2–3 times to initiate the spread. Return to the oven to finish baking.
Cool cookies for 5 minutes on the baking sheet, then transfer to a cooling rack to cool completely. The cookies will slightly deflate as they cool.
Cookies stay fresh covered at room temperature for up to 1 week. Enjoy the zesty delight of lemon crinkle cookies, perfect for any time of year!
