Preheat and prep the pan: Heat your oven to 325°F (165°C).
Line a 9x13-inch metal baking pan with parchment, leaving an overhang for easy lifting. Lightly grease the parchment.
Make the crust: In a bowl, whisk the melted butter, sugar, salt, and vanilla until combined. Stir in the flour until a soft dough forms.
It should look thick and slightly crumbly but hold together when pressed.
Press and bake the crust: Press the dough evenly into the prepared pan, pushing it slightly up the sides. Prick the surface lightly with a fork. Bake for 18–20 minutes, until the edges are lightly golden.
Keep the oven at 325°F.
Whisk the filling: While the crust bakes, whisk sugar and flour in a bowl. Add eggs and whisk just until smooth. Stir in lemon juice, zest, and a pinch of salt.
Don’t overwhisk—you don’t want too many bubbles.
Pour and bake: Give the filling a quick stir, then pour it over the hot crust. Return to the oven and bake for 18–22 minutes, or until the center is set and no longer jiggly. A slight wobble is okay; it will firm up as it cools.
Cool completely: Let the bars cool in the pan on a rack until room temperature.
Then chill for at least 2 hours for the cleanest slices.
Finish and slice: Lift the slab from the pan using the parchment. Dust generously with powdered sugar. Use a sharp knife to cut into squares or rectangles, wiping the blade between cuts for neat edges.