Preheat your oven to 350°F (175°C). Line a 9×13 inch baking pan with parchment paper, leaving an overhang on the sides. This makes it easy to lift the bars out later for clean slicing.
In a medium bowl, combine 2 cups of flour, 1/2 cup of sugar, and the salt. Add the cold, cubed butter. Use a pastry cutter or your clean fingers to blend the butter into the dry ingredients until the mixture resembles coarse crumbs. Press the mixture firmly into the bottom of your prepared baking pan.
Place the pan in the oven and bake for 20 minutes, or until the edges are just barely starting to turn a light golden brown. Removing the crust while it is still pale ensures it stays tender rather than becoming hard.
While the crust bakes, whisk the 6 eggs and 2 1/2 cups of sugar together in a large bowl until smooth. Stir in the lemon zest and fresh lemon juice. Finally, sift in the remaining 1/2 cup of flour and whisk until no lumps remain.
Once the crust is ready, pour the lemon mixture directly over the warm crust. Return the pan to the oven and bake for another 30 to 35 minutes. The filling should be set and no longer jiggle in the center when moved.
Remove the pan from the oven and let it cool completely to room temperature on a wire rack. For the best texture, refrigerate the bars for at least 2 hours before slicing. Just before serving, dust the top generously with sifted powdered sugar.