Prepare your baking dish. Preheat your oven to 350°F (175°C). Line an 8×8 or 9×9 inch baking pan with parchment paper, leaving some overhang on the sides to easily lift the bars out later.
Make the shortbread crust. In a medium bowl, whisk together flour, sugar, and salt. Add the cold butter cubes and use your fingers or a pastry cutter to work the butter into the dry ingredients until the mixture resembles coarse crumbs.
Press and pre-bake the crust. Pour the crumbly mixture into your prepared pan and press firmly into an even layer. Bake for 15-18 minutes until the edges are just beginning to turn golden.
Prepare the lemon filling while the crust bakes. In a medium bowl, whisk the eggs, sugar, flour, and salt until smooth. Add the lemon juice and zest, whisking until well combined.
Pour the filling over the hot crust. When the crust is done pre-baking, remove it from the oven and immediately pour the lemon filling over the hot crust.
Bake until set. Return the pan to the oven and bake for an additional 18-22 minutes, or until the filling is set and doesn’t jiggle when you gently shake the pan.
Cool completely before cutting. Allow the bars to cool at room temperature for at least 1 hour, then refrigerate for at least 2 hours or overnight for the cleanest cuts.
Finish and serve. Lift the bars out using the parchment paper overhang, dust generously with powdered sugar, and cut into 16 squares. For the cleanest cuts, wipe your knife clean between slices.