Heat the oven to 350°F and line an 8-inch square baking pan with parchment, leaving enough paper hanging over two sides to lift the bars later. Lightly grease the exposed sides of the pan so the crust releases cleanly.

Stir the flour, sugar, and salt together in a bowl, then mix in the melted butter until the mixture resembles damp sand. Press it firmly and evenly into the prepared pan, using the bottom of a measuring cup to create a compact surface.

Bake the crust for 18 to 20 minutes, until the edges are lightly golden and the center looks set. Let it cool for about 5 minutes while you prepare the filling, but do not cool it completely.

Whisk the eggs and sugar in a clean bowl until smooth, then add the lemon juice, lemon zest, flour, and salt. Whisk just until combined; excessive beating can add air bubbles that show on the finished surface.

Pour the lemon mixture over the warm crust and return the pan to the oven. Bake for 15 to 18 minutes, until the filling is set around the edges and has only a slight wobble in the middle.

Cool the pan on a rack for 1 hour, then refrigerate the bars for at least 2 hours before cutting. Lift them out with the parchment, dust with powdered sugar, and slice into 16 squares with a sharp knife.