For the crust, cream the butter and sugar in the bowl of an electric mixer with a paddle attachment. Combine the flour and salt and add to butter mixture. Line a 9 X 13" pan with aluminum foil and spray with nonstick cooking spray. Put the dough into the pan and sprinkle with flour and using a small roller or your hands, roll into the pan building up a 1/2 inch edge on all sides.
Put pan in the freezer for 10 minutes before baking
Bake the crust for 20 minutes, until lightly browned. Make the filling while the crust is baking. Remove from the oven.
For the filling, whisk together the eggs, sugar, lemon zest, lemon juice and flour. Using real lemons makes all the difference. Mix again just before you pour over the crust and bake for 30 minutes, until filling is set. Let cool to room temperature. Cut into 20 - 24 squares or 40 triangles. It's easier to cut if you refrigerate for 30 minutes after completely cool.
Notes
After you bake the crust, freeze the baking pan for 10 minutes.The filling should be poured into the warm crust right after baking. Prepare the filling while the crust is in the freezer. The baking time may vary for the filling. Make sure your oven is calibrated and that you understand your baking time based on your oven settings. Do not underbake the filling or the bars won't hold up. Sift powdered sugar on bars right before consuming. When slicing, make sure the bars are cold and use a clean sharp knife. I cut all the edges off so that the bars are all pretty. Then I eat the edges. Yum.