Step 1

Arrange a rack in the middle of the oven and heat to 350ºF. Lightly coat the bottom and sides of a 9x13-inch glass baking dish with butter. Line the bottom and two long sides with a sheet of parchment paper long enough to hang over the sides by 1-inch to form a sling.

Step 2

Melt 12 tablespoons unsalted butter in the microwave or on the stovetop. Break 24 full-size graham crackers into large pieces and place in a food processor fitted with the blade attachment. Add 1/4 cup granulated sugar and 1/4 teaspoon kosher salt. Pulse until fine crumbs form, about 15 (1-second) pulses.

Step 3

Transfer the crumbs to a large bowl, drizzle with the butter, and stir until combined and the crumbs are evenly moistened. Transfer the mixture into the baking dish and press with your hands or the bottom of a measuring cup into an even layer.

Step 4

Bake until fragrant and starting to darken around the edges, 12 to 15 minutes. Place on a wire rack and keep the oven on at 350ºF.

Step 5

Finely grate the zest of 1 to 2 medium lemons until you have 1 tablespoon plus 2 teaspoons. Juice the zested lemons plus more as needed until you have 1 cup.

Step 6

Place 6 large egg yolks in the bowl of a stand mixer (or large bowl if using an electric hand mixer). Beat with the whisk attachment on low speed until combined, about 15 seconds. With the mixer still on low speed, gradually pour in 2 (14-ounce) cans sweetened condensed milk. Gradually increase the speed to medium and beat until thickened, about 2 minutes.

Step 7

Add the lemon juice, 1 tablespoon of the lemon zest (cover and refrigerate the remaining for the topping), and 1/2 cup limoncello and beat until just combined, about 20 seconds. Pour the mixture into the crust.

Step 8

Bake until the filling is just set in the middle, 20 to 25 minutes. Place the baking dish on a wire and let cool completely, about 1 hour. Refrigerate for at least 4 hours or up to overnight. Make the topping right before serving.

Step 9

Place the reserved lemon zest, 1 cup cold heavy cream, 2 tablespoons powdered sugar, and 1 teaspoon vanilla extract in the bowl of a stand mixer (or large bowl if using an electric hand mixer). Beat with the whisk attachment on medium speed for 1 minute. Increase the speed to high and beat until stiff peaks form, 3 to 4 minutes.

Step 10

Grasping the excess parchment, lift the slab out of the baking dish and place on a cutting board. Use a knife to score into 24 bars. Transfer the topping to a piping bag fitted with a large star tip and pipe rosettes onto each bar, or use a spoon to dollop the cream onto each bar. Fully slice and serve.