From Cook's Illustrated - May/June, 1998
Crust
1 3/4 cups all-purpose flour 3/4 teaspoon salt
2/3 cup confectioners' sugar, plus extra to decorate 12 tablespoons unsalted butter (1 sticks), cold, cut into 1-inch pieces, plus extra for greasing pan
Filling
1/4 cup cornstarch
4 large eggs, beaten lightly 2/3 cup juice from 3 to 4 large lemons, strained
1 1/3 cups granulated sugar (I usually use no more than 1 cup and have great results.) 1/3 cup whole milk
3 tablespoons all purpose flour 1/8 teaspoon salt
2 teaspoons finely grated zest from two large lemons (Don't worry about measuring, use the zest of two lemons; just the yellow, no white pith)
For the crust: Adjust oven rack to middle position and heat oven to 350 degrees. Lightly butter a 13-by-9-inch baking dish and line with one sheet parchment or wax paper. Dot paper with butter, lay second sheet crosswise over it. Be sure to leave extra paper up the sides of the pan. (You are crossing the two sheets of paper, which will become the means of lifting the lemon bars out of the pan in one piece. This is in order to avoid the bars falling apart in the pan if cut in the pan.)
Pulse flour, confectioners' sugar, cornstarch, and salt in food processor workbowl 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-secornd bursts. (To do this by hand, mix flour, confectioners' sugar, cornstarch, and salt in medium bowl. Keep butter very cold and when ready to use cut into very small pieces and toss the butter bits in the flour to coat. Rub pieces between your fingers for a minute, until flour turns pale yellow and coarse.) Remember, the mixture is not dough, it is a coarse flour mixture. Sprinkle mixture into lined pan and press firmly with fingers into even, 1/4-inch layer over entire pan bottom and about -inch up sides. Refrigerate for 30 minutes, then back until golden brown, about 20 minutes.
For the filling: Meanwhile, whisk eggs, sugar and flour in medium bowl, then stir in lemon juice, zest, milk, and salt to blend well.
To finish the bars: To finish the bars: Reduce oven temperature to 325 degrees. 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 at least 30 minutes. Remove the bars from the pan by grasping the parchment/wax paper firmly by the lengthwise edges of the pan and lifting the bars onto a cutting board. Fold the paper down off the edges, and cut into serving-size bars, wiping knife or pizza cutter clean between cuts, as necessary. Sieve confectioners' sugar over bars, if desired.