Step 1

Preheat the oven to 350 degrees F. Line a 9-by-13-inch baking pan with foil, leaving an overhang on 2 sides; spray lightly with nonstick cooking spray. Set aside.

Step 2

For the crust: Add the flour, cornstarch, confectioners' sugar and salt to the bowl of a food processor and pulse 10 times until combined. Add the cold butter and pulse until the mixture looks like coarse sand, about 30 times. (The dough will not come together in one mass.) Sprinkle the mixture evenly in the prepared pan and lightly pat it down. Bake, rotating the pan halfway through, until golden brown around the edges, 20 to 25 minutes.

Step 3

For the filling: Combine the eggs, yolks, sugar, flour, salt and lemon juice in a medium saucepan. Whisk continuously over medium heat, occasionally scraping down the sides and edges of the pan with a rubber spatula, until simmering and thickened, 4 to 5 minutes. Remove from the heat and whisk in the lemon zest and butter until combined.

Step 4

Lower the oven temperature to 300 degrees F. Pour the filling over the crust and bake until just set, 12 to 15 minutes.

Step 5

Place on a wire rack to cool to room temperature, then refrigerate until cold, about 2 hours.

Step 6

Use the foil overhang to lift the bars from the pan. Remove the foil and slice into squares. Dust with confectioners' sugar just before serving.