Preheat oven to 325 degrees F. Line an 8”x8” baking pan, preferably glass, with parchment paper and set aside.

To make the crust, add melted butter, sugar, vanilla, and salt to a medium bowl; stir to combine. Add flour and stir until thoroughly incorporated. The dough will be very thick. Use your fingertips to press the dough firmly and evenly across the bottom of the prepared pan. Bake for 16-18 minutes, just until the edges are very lightly browned.

Meanwhile, prepare the filling. In the same bowl, whisk together the sugar and flour. In a large liquid measuring cup, whisk together the lemon juice and eggs, then pour them into the sugar-flour mixture. Whisk once again, vigorously, until everything is evenly combined.

Pour filling over the warm crust. If there are any large air bubbles, pop them very gently with a toothpick. Return pan to the oven for about 20 minutes, just until the center is set and doesn’t jiggle when you tap the pan lightly. Remove from the oven and cool for about 1 hour at room temperature, then refrigerate for another 2-3 hours to set completely.

When the bars are fully chilled, use the parchment edge to lift them out of the pan and set on a cutting board. Dust with powdered sugar, if desired, and cut into squares. Serve chilled.