It's tough to top a classic lemon bar, but we've done it! We kept the tender, buttery crust and bright filling of the original, and made a few improvements. For our luscious lemon layer, we employed a unique double-cooking method. Simmering first on the stovetop like lemon curd allows us to add butter, which balances the tart and sweet elements and adds a richer, smoother texture.
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.
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.
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.
Lower the oven temperature to 300 degrees F. Pour the filling over the crust and bake until just set, 12 to 15 minutes.
Place on a wire rack to cool to room temperature, then refrigerate until cold, about 2 hours.
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.
When measuring flour, we spoon it into a dry measuring cup and level off excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)