1/2 cup confectioners’ sugar, plus more to decorate the finished bars
1/2 teaspoon salt
8 tablespoons unsalted butter, softened but still cool, cut into 1-inch pieces
For the lemon filling
7 large egg yolks, plus 2 large eggs
1 cup plus 2 tablespoons granulated sugar
2/3 cup from 4 or 5 medium lemons, plus 1/4 cup finely grated zest
Pinch of salt
4 tablespoons unsalted butter, cut into 4 pieces
3 tablespoons heavy cream
Make the crust
1. Spray a 9-inch square baking pan with nonstick cooking spray.
Fold two 16-inch pieces of foil or parchment paper lengthwise to measure 9 inches wide. Fit one sheet in the bottom of the greased pan, pushing it into the corners and up the sides of the pan (overhang will help in removal of baked bars). Fit the second sheet in the pan in the same manner, perpendicular to the first sheet. Spray the sheets with nonstick cooking spray.
2. Place the flour, confectioners’ sugar, and salt in a food processor and process briefly. Add the butter and process to blend, 8 to 10 seconds, then process until the mixture is pale yellow and resembles coarse meal, about three 1-second pulses.
3. Adjust an oven rack to the middle position and heat the oven to 350°F. Bake the crust until golden brown, about 20 minutes.
Make the filling
1. In a medium non-reactive bowl, whisk together the yolks and whole eggs until combined, about 5 seconds.
Add the lemon juice, zest, and salt; whisk until combined, about 5 seconds.
Transfer the mixture to a medium non-reactive saucepan, add the butter pieces, and cook over medium-low heat, stirring constantly with a wooden spoon, until the curd thickens to a thin sauce-like consistency and registers 170°F on an instant-read thermometer, about 5 minutes.
Remove the bars from the pan using the foil or parchment handles and transfer to a cutting board. Cut into 2 1/2 inch squares, wiping the knife clean between cuts as necessary. Sieve confectioners’ sugar over the bars, if desired.