Method
- Preheat the oven to 350 F. Line a 9-inch square baking pan with parchment paper, leaving an overhang on two sides for easy removal.
- For the crust, pulse 1 cup flour, 1/4 cup powdered sugar, and 1/4 tsp salt in a food processor, or whisk together in a bowl. Add 1/2 cup cold cubed butter and pulse or cut in with a fork until the mixture looks like coarse, sandy crumbs.
- Press the mixture firmly and evenly into the bottom of the prepared pan using the bottom of a measuring cup. Bake for 18 to 20 minutes, until light golden at the edges. A fully baked crust is what keeps the bottom from turning soggy under the wet lemon filling.
- While the crust bakes, whisk the 4 eggs, 1 1/2 cups granulated sugar, and 1/3 cup flour together in a bowl until smooth with no lumps of flour.
- Whisk in 2/3 cup fresh lemon juice and 1 tbsp lemon zest until fully combined. Using fresh-squeezed juice rather than bottled makes a noticeable difference in brightness.
- Pour the lemon filling over the hot, freshly baked crust right after it comes out of the oven; pouring it while the crust is hot helps the layers bond together.
- Return the pan to the oven and bake for 22 to 25 minutes, until the filling is set and no longer jiggles in the center when the pan is gently shaken, but has not browned.
- Cool the bars completely in the pan on a wire rack, about 1 hour, then refrigerate for at least 2 hours to fully firm up before cutting - cutting warm will cause the filling to smear instead of holding a clean edge.
- Lift the bars out using the parchment overhang, dust the top generously with 2 tbsp powdered sugar, and cut into 16 squares with a sharp knife, wiping the blade clean between cuts.
Tips
Roll the lemons on the counter under your palm before juicing to get more juice out of each one.
Chilling the bars fully before cutting is the single biggest factor in getting clean, bakery-style squares instead of a runny mess.