Heat the oven to 325°F (163°C), then line the bottom and sides of a 9×13-inch glass baking pan with parchment, leaving overhang on two sides. The parchment sling should sit flat against the corners and extend high enough to grip after chilling.

Stir the melted butter, sugar, vanilla extract, and salt in a medium bowl, add the flour, and mix until a thick dough forms; press it firmly and evenly into the prepared pan. Bake for 20-22 minutes, until the edges turn lightly golden and the kitchen smells warmly buttery.

Prick the warm crust all over with a fork without piercing through to the pan. These shallow holes give the lemon layer places to grip, helping prevent the crust from sliding away from the filling.

Sift the sugar and flour into a large bowl, whisk in the eggs, then stir in the fresh lemon juice and optional lemon zest until smooth. The mixture should look fluid and evenly blended, without floury pockets or large frothy bubbles.

Pour the filling over the warm docked crust and bake for 22-26 minutes. The center should look mostly set and move with a small, even wobble when the pan is gently tapped with an oven mitt.

Cool the pan completely at room temperature for about 2 hours, then refrigerate for 1-2 hours until thoroughly chilled before lifting out, dusting, and cutting. Wipe the knife clean between cuts for sharp squares, then cover leftovers and refrigerate for up to 1 week.

For freezing, cut fully cooled bars without confectioners’ sugar, arrange them on a baking sheet, and freeze for 1 hour before wrapping each square in aluminum foil or plastic wrap and placing them in a freezer container. Keep them frozen for up to 3-4 months, thaw in the refrigerator, and add confectioners’ sugar just before serving.