Lemon Bars

From America's Test Kitchen, here are their notes:

For lemon bars with sweet-tart flavor and a crisp, well-browned crust, we started at the bottom. Our pat-in-the-pan crust is made with meltedónot coldóbutter and can therefore be stirred together instead of requiring a food processor. For a truly crisp texture, we used granulated sugar instead of the usual confectioners' sugar and baked the crust until it was dark golden brown to ensure that it retained its crispness even after we topped it with the lemon filling.

We cooked our lemon filling on the stove to shorten the oven time and keep it from curdling or browning at the edges when it baked. The combination of lemon juice and lemon zest provides complex flavor, and a couple of teaspoons of cream of tartar (tartaric acid) give it a bright, lingering finish.

Ingredients

Crust

1 cup (5 ounces) all-purpose flour
1/4 cup (1 3/4 ounces) granulated sugar
1/2 teaspoon salt
8 tablespoons unsalted butter, melted

Filling

1 cup (7 ounces) granulated sugar
2 tablespoons all-purpose flour
2 teaspoons cream of tartar
1/4 teaspoon salt
3 large eggs plus 3 large yolks
2 teaspoons grated lemon zest plus 2/3 cup juice (4 lemons)
4 tablespoons unsalted butter, cut into 8 pieces

Confectioners' sugar (optional)

Instructions

1. FOR THE CRUST: Adjust oven rack to middle position and heat oven to 350 degrees. Make foil sling for 8-inch square baking pan by folding 2 long sheets of aluminum foil so each is 8 inches wide. Lay sheets of foil in pan perpendicular to each other, with extra foil hanging over edges of pan. Push foil into corners and up sides of pan, smoothing foil flush to pan.

2. Whisk flour, sugar, and salt together in bowl. Add melted butter and stir until combined. Transfer mixture to prepared pan and press into even layer over entire bottom of pan (do not wash bowl). Bake crust until dark golden brown, 19 to 24 minutes, rotating pan halfway through baking.

3. FOR THE FILLING: While crust bakes, whisk sugar, flour, cream of tartar, and salt together in now-empty bowl. Whisk in eggs and yolks until no streaks of egg remain. Whisk in lemon zest and juice. Transfer mixture to saucepan and cook over medium-low heat, stirring constantly, until mixture thickens and registers 160 degrees, 5 to 8 minutes. Off heat, stir in butter. Strain filling through fine-mesh strainer set over bowl.

4. Pour filling over hot crust and tilt pan to spread evenly. Bake until filling is set and barely jiggles when pan is shaken, 8 to 12 minutes. (Filling around perimeter of pan may be slightly raised.) Let bars cool completely, at least 1½ hours. Using foil overhang, lift bars out of pan and transfer to cutting board. Cut into bars, wiping knife clean between cuts as necessary. Before serving, dust bars with confectioners' sugar, if using.