Adjust an oven rack to middle position and preheat the oven to 350°F. Line a 9x13-inch baking pan with one sheet parchment paper.

Pulse flour, confectioners' sugar, cornstarch, and salt in a food processor. Add the butter and process for 8 to 10 seconds and then pulse until the mixture is pale yellow and resembles coarse meal, about three 1-second pulses. (To do this by hand, mix flour, confectioners' sugar, cornstarch, and salt in medium bowl. Freeze the butter and grate it on the large holes of a box grater into the flour mixture. toss the butter pieces to coat. Rub the pieces between your fingers for a minute, until flour turns pale yellow and coarse.) sprinkle the mixture into the lined pan and press firmly with fingers into an even ¼-inch layer over the entire pan bottom and ½-inch up the sides. Refrigerate for 30 minutes, then bake until golden brown, about 20 minutes.

Whisk the eggs, sugar and flour in a medium bowl, then stir in the lemon zest, lemon juice, milk and salt to blend well.

Reduce the oven temperature to 325°F. Stir the filling mixture to reblend; pour into warm crust. Bake until the filling feels firm when touched lightly, about 20 minutes. Transfer the pan to a wire rack; cool to near room temperature, at least 30 minutes. Transfer to cutting board, fold paper down, and cut into bars. Sift confectioners' sugar over bars, if desired. Leftover bars can be covered in plastic wrap and refrigerated for up to 2 days.