1¾ cups all-purpose flour
⅔ cup confectioners' sugar, plus extra to decorate finished bars
¼ cup sornstarch
¾ teaspoon salt
12 tablespoons unsalted butter, softened and cut into 1-inch pieces
4 large eggs, beaten lightly
1⅓ cups granulated sugar
3 tablespoons all-purpose flour
2 teaspoons finely grated zest from two large lemons
⅔ cup juice from 3 to 4 large lemons, strained (I ended up having ½ cup, and the bars were still very lemony)
⅓ cup whole milk
⅛ teaspoon salt
For the crust:
Adjust oven rack to middle position and heat oven to 350ºF. Lightly butter a 13x9-inch baking dish and line with foil, using this method.
Pulse flour, confectioners' sugar, cornstarch, and salt in workbowl of food processor fitted with steel blade. Add butter and process to blend, 8 to 10 seconds, then pulse until mixture is pale yellow and resembles coarse meal, about three 1-second bursts. (To do this by hand, mix flour, confectioner's sugar, cornstarch, and salt in medium bowl. Freeze butter and grate it on large holes of box grater into flour mixture. Toss butter pieces to coat. Rub pieces between your fingers for a minute, until flour turns pale yellow and coarse.) Sprinkle mixture into lined pan, pressing firmly with fingers into even, ¼-inch layer over entire pan bottom and about ½ inch up sides. Refrigerate for about 30 minutes, then bake until golden brown, about 20 minutes.
For the filling:
Meanwhile, whisk eggs, sugar and flour in medium bowl, then stir in lemon zest, lemon juice, milk, and salt to blend well.
To finish:Reduce oven temperature to 325ºF. Stir filling mixture to reblend; pour into warm crust. Bake until filling feels firm when touched lightly, about 20 minutes. Transfer pan to wire rack; cool to near room temperature, at least 30 minutes. Grasp edges of the lengthwise piece of parchment or waxed paper and lift the bars onto a cutting board. Fold paper down and cut into serving-size bars, wiping knife or pizza wheel clean between cuts, as necessary. Sieve confectioners' sugar over bars, if desired.