The solution: We tackled the crust first, knowing that our basic ingredients would be flour, butter, and sugar and that the outcome should be more like a cookie or a shortbread than a pastry-type crust. This type of crust generally calls for a fair amount of sugar and no water (which is wanted in pastry crusts to create pockets of steam that produce flakiness and layering). White granulated sugar is often the first bakers turn to for the sort of crust we were after, but we discovered that confectioners’ sugar gave us the most tender texture. The addition of a little cornstarch also helped move the crust in the melt-in-your-mouth direction. To ensure an evenly baked crust and prevent sogginess, we found it necessary to prebake before adding the filling.
To make the filling lemony enough to satisfy us, we ended up using the juice from three or four lemons, plus some zest. Arriving at a smooth and pleasant texture involved eggs, a little flour for thickening, and, somewhat unexpectedly, milk, which seemed to balance the flavor with the texture.
We were now left with only one problem to solve: cutting and serving the bars cleanly. In all of our tests, the filling inevitably shifted around and stuck to the sides of the pan, making it difficult to cut the bars around the edges. Our solution was to line the pan with paper (parchment or waxed is fine), which made it possible for us to lift the baked lemon bars, uncut, out of the pan and cut them without interference from the sides of the pan.

PERFECT LEMON BARS

Makes about two dozen 1 1/2- to 2-inch squares

The lemon filling must be added to a warm crust. The 30-minute chilling and 20-minute baking of the crust should allow plenty of time to prepare the filling. If not, make the filling first and stir to blend just before pouring it into the crust. Any leftover bars can be sealed in plastic wrap and refrigerated for up to two days.

THE CRUST
1 3/4 cups all-purpose flour
2/3 cup confectioners’ sugar, plus extra to decorate finished bars
1/4 cup cornstarch
3/4 teaspoon salt
12 tablespoons unsalted butter (1 1/2 sticks), at very cool room temperature, cut into 1-inch
pieces, plus extra for greasing pan

LEMON FILLING
4 large eggs, beaten lightly
1 1/3 cups granulated sugar
3 tablespoons all-purpose flour
2 teaspoons finely grated zest from two large lemons
2/3 cup juice from 3 to 4 large lemons, strained
1/3 cup whole milk
1/8 teaspoon salt

1. 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, then lay second sheet crosswise over it.

2. 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-second bursts. (To do this by hand, mix flour, confectioners’ 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 and press firmly with fingers into even, 1/4-inch layer over entire pan bottom and about 1/2-inch up sides. Refrigerate for 30 minutes, then bake until golden brown, about 20 minutes.

3. For the filling: Meanwhile, whisk eggs, sugar, and flour in medium bowl, then stir in lemon juice, zest, milk, and salt to blend well.

4. 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 to near room temperature, at least 30 minutes.Transfer to cutting board, fold paper down, and cut into serving-size bars, wiping knife or pizza cutter clean between cuts, as necessary. Sieve confectioners’ sugar over bars, if desired.