Prepare the baking dish

Lightly butter a 9 x 13-inch baking dish with 2 teaspoons butter and line with 1 sheet of parchment or wax paper. Butter the paper with 1 teaspoon of the remaining butter and then lay a second sheet of parchment or wax paper crosswise over the first sheet. This parchment sheet should be large enough for the ends to be even with the tops of the long sides of the baking dish-this extra paper will function as handles to help you remove the lemon squares from the pan later. Set the dish aside.

Make the crust

In a large bowl, combine 1 3/4 cups flour, the confectioners' sugar, the cornstarch, and 3/4 teaspoon salt and mix thoroughly. Cut the remaining 1 1/2 sticks butter into small pieces and add to the flour mixture. Using your hands, two forks, or a pastry blender, work the butter into the flour mixture until the mixture resembles coarse meal. Transfer the butter-flour mixture to the prepared baking dish and press into an even 1/4-inch layer along the bottom and partly up the sides of the dish. Refrigerate for 30 minutes. While the crust is chilling, preheat the oven to 350 degrees F.

Bake the crust until golden brown, 20 to 25 minutes.

Make the filling

Meanwhile, combine the eggs, granulated sugar, the remaining 3 tablespoons flour, and lemon zest in a medium bowl and whisk until smooth. Stir in the lemon juice, milk, Limoncello, and remaining pinch salt and mix well.

When the crust is golden brown, remove it from the oven and reduce the temperature to 325 degrees F. Stir the lemon mixture again, then pour over the warm crust. Bake until the filling is set, about 20 minutes. Transfer to a wire rack to cool completely. Grasp the extra paper that lines the two longest sides of the baking dish and remove the bars from the pan by pulling up gently. The entire dessert should easily dislodge and come away from the pan. Transfer to a cutting board and, using a clean knife, cut into squares, wiping the knife after each cut. Place a small amount of confectioners' sugar in a small sieve, and sprinkle the bars with the sugar. Serve immediately, or refrigerate, wrapped with plastic wrap, for up to two days, or until ready to serve.