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 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 up the bottom of the bowl. Add the flour mixture and beat on low speed until combined. Dough is thick, creamy, and sticky.

Cover dough tightly and chill in the refrigerator for at least 3 hours and up to 3 days. Chilling is mandatory for this sticky cookie dough.

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

Preheat 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 of dough each, into balls. Some readers have had trouble with the confectioners’ sugar melting/absorbing into the cookie dough balls, so to prevent that, you can roll the balls in the granulated sugar first. And then roll each ball very generously in the confectioners’ sugar.

Bake the cookies for 12–13 minutes or until the edges appear set and the centers still look soft. If they aren’t really spreading by minute 9, remove them from the oven and lightly bang the baking sheet on the counter 2–3x. This helps initiate that spread. Return to the oven to continue baking.

Cool cookies for 5 minutes on the baking sheet, and 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.