Prep and Preheat: Start by preheating your oven to 350°F (175°C). Line an 8×8 inch baking pan with parchment paper, leaving an overhang on two sides for easy removal.

Make the Crust: In a medium bowl, whisk together the flour, ¼ cup powdered sugar, and salt. Add the cold, cubed butter. Use a pastry cutter, a fork, or your fingertips to work the butter into the flour until the mixture resembles coarse crumbs and holds together when pressed.

Par-Bake the Crust: Press the crust mixture firmly and evenly into the bottom of your prepared pan. Use a flat-bottomed glass to get it really compact. Bake for 18-20 minutes, or until the edges are just barely golden.

Prepare the Filling: While the crust is baking, make the filling. In a medium bowl, whisk together the lemon juice, lemon zest, and eggs. In a separate small bowl, whisk the granulated sugar and ⅓ cup flour together. This prevents lumps. Add the sugar-flour mixture to the wet ingredients and whisk until completely smooth.

Assemble and Bake: Once the crust is out of the oven, immediately pour the lemon filling over the hot crust.

Bake to Set: Return the pan to the oven and bake for another 20-25 minutes, until the filling is set and no longer jiggly in the center.

Cool Completely: Let the pan cool completely on a wire rack, then refrigerate for at least 2 hours (or overnight) to fully set before slicing.

Serve: Just before serving, use a fine-mesh sieve to dust the top with a thick, even layer of powdered sugar. Slice into bars.