Put granulated sugar into large bowl. If using vanilla beans, cut in half lengthwise and scoop the pulp over the sugar. Add the lemon zest. Using your fingertips, rub the vanilla pulp and lemon zest into the sugar until it's fragrant. (If using vanilla extract, you'll add it later.)

Add the butter, confectioners' sugar, and salt to the bowl, and beat on low speed just until the mixture is smooth and creamy. You don't want it to become light and fluffy.

Add the egg yolk, and vanilla extract if using. Beat for 1 minute on medium speed.

Add the flour; mix on low speed just until the flour is incorporated.

Transfer the soft, sticky dough to a gallon-size ziploc bag. Put the bag on a flat surface. Leaving the top open, roll the dough into a ¼-inch thick rectangle that completely fills the bag. As you roll, turn the bag occasionally and lift the plastic from the dough so that it doesn’t cause creases. Seal the bag, and refrigerate the dough for at least 2 hours, or for up to 2 days.

Preheat the oven to 325° F. Line two baking sheets with parchment or silicone mats.

Put the ziploc bag on a cutting board and slit it open. Turn the firm dough onto a parchment lined board or counter; discard the bag. Using a ruler as a guide and a sharp knife or pastry cutter, cut the dough into 1½-inch squares. Transfer the squares to the baking sheets and carefully prick each one twice with a fork, gently pushing the tines through the cookies until they hit the baking sheet. Refrigerate for 20 minutes.

Bake at 325° F for 18 to 20 minutes, rotating the sheets from the top to bottom and front to back at the midway point, until the cookies are lightly browned around the edges, and golden on the bottom. Transfer the cookies to a rack to cool.

Store in airtight container for up to a week. Yield: 4-dozen.