Line an 8x8-inch square baking pan with 2 sheets of parchment paper, positioning them perpendicular to each other so that the parchment hangs about 2 inches over all 4 sides of the pan to form a sling.

Break 12 whole graham cracker sheets into rough pieces and place in a food processor. Add 2 tablespoons granulated sugar and ½ teaspoon kosher salt. Process until the crackers are the texture of sand, about 15 seconds. Remove 2 tablespoons of the graham cracker crumbs and reserve for garnish.

Add 1 stick melted and cooled unsalted butter to the food processor and pulse to combine, 6 to 8 (1-second) pulses.

Transfer the mixture into the baking pan. Use the bottom of a flat measuring cup or your fingers to press the crumbs into an even layer, making sure to reach the edges and fill the corners.

Whisk 1 (14-ounce) can sweetened condensed milk, 2 tablespoons of the finely grated lemon zest, ½ cup lemon juice, and ¼ teaspoon kosher salt together in a large bowl.

Beat 1 ½ cups cold heavy cream in a stand mixer with the whisk attachment on medium speed until soft peaks form, 3 to 4 minutes. Fold into the lemon mixture with a flexible spatula until no streaks remain.

Transfer onto the crust and smooth into an even layer. Sprinkle the reserved graham cracker crumbs and remaining 1 tablespoon lemon zest evenly over the top. Cover and freeze until firm, at least 3 hours or up to overnight.

Grasping the excess parchment, lift the slab out of the pan and place on a cutting board. Cut into 12 bars, wiping the knife clean between each cut for the cleanest-looking slices.