Preheat the oven to 325°F / 165°C. Line a 9 × 13in / 23 × 33cm glass baking dish with parchment paper.
Mix 1 cup of melted unsalted butter, ½ cup of granulated sugar, 2 teaspoons of vanilla extract, and ½ teaspoon of salt. Stir in 2⅛ cups of all-purpose flour until combined, then press evenly into the dish.
Bake for 20-22 minutes, until the edges are lightly browned. Prick the warm crust all over with a fork, without going all the way through.
While the crust bakes, sift 2 cups of granulated sugar with 6 tablespoons of all-purpose flour. Whisk in 6 large eggs and 1 cup of fresh lemon juice.
Pour the filling over the warm crust. Bake for about 20 minutes, until the center is set and no longer jiggles.
Cool at room temperature, then refrigerate until chilled. Cut into bars.
