In a small bowl, whisk together the flour, cornstarch, and Kosher salt. Pour a few Tablespoons of granulated sugar into a shallow bowl. Set aside. Then, line 2 sheet pans with parchment paper.

Pour the sugar and lemon zest into a large mixing bowl. Using clean hands, "rub" the sugar and lemon zest together until it's very fragrant and the lemon zest has released its oils, about 1-2 minutes.

Add the butter to the lemon sugar. Using a hand mixer or stand mixer, beat the butter and sugar together until it's pale and fluffy, about 3-4 minutes. Mix in the egg yolks and vanilla bean paste.

Add the flour mixture and gently beat the dough. It will be crumbly. Once you have large chunks of dough, use a rubber spatula to mix the dough just until it's smooth and combined.

Using a #60 cookie scoop (1 tablespoon), scoop the dough into balls. Roll the cookie dough balls until they're very smooth and then roll the cookie dough balls in the bowl of granulated sugar.

Place the cookie dough balls about 2 inches apart on the prepared sheet pans. (You should be able to fit 12 cookies to a pan.) Using the bottom of a teaspoon, evenly press an indent into the center of each cookie. Repeat with the remaining cookie dough.

Transfer the prepared cookies to the freezer or refrigerator. Freeze the cookie dough for 30 minutes or refrigerate for 1 hour. Meanwhile, preheat the oven to 350°F (177°C).

Once the dough is chilled, remove it from the fridge or freezer. Fill a piping bag with lemon curd. Then, fill the center of each cookie with about 1/2 teaspoon of lemon curd. (You can also use a teaspoon to spoon the lemon curd into the centers of the cookies.)

Bake the cookies for 11-12 minutes, until the bottoms of the cookies are a light golden-brown and the filling is set. (It may bubble up in the oven, but it should not overflow.) Let the cookies cool on the sheet pan for about 5 minutes. Then, transfer the cookies to a wire rack to cool completely. Transfer the cookies to an airtight container and refrigerate for up to 5 days. Enjoy!