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 6 eggs, 3 cups sugar, 2 Tablespoons lemon zest, 3/4 cups lemon juice and 1 cup 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
✔ Always use fresh lemons.✔ Chill before slicing.✔ Dust with powdered sugar immediately before serving.✔ Wipe the knife clean between cuts.✔ Bake until the center barely jiggles.✔ Freeze individually for easy desserts later.