Preheat oven to 350 degrees F. Line a 13x9" baking pan with foil, extending the sides of the foil over the edges of the pan. Spray the foil lightly with cooking spray; set aside.
In a large bowl, stir together the flour, sugar and salt. Cut in the butter cubes with a pastry blender or two forks until coarse crumbs form. To test if it's done, grab a fistful of the crust dough; if it holds together in one piece, it's done.
Pour the crust into the prepared pan and pat it into an even layer. Bake for approx. 7-8 minutes or until lightly golden brown.
While the crust bakes, make your filling. In another medium bowl, whisk together the flour and sugar together. Whisk it really well so all those clumps of flour are broken up. Alternatively, you may sift the flour into the sugar mixture. Add in the eggs, lemon juice and lemon zest and stir to combine. Pour the mixture over the crust.
Bake for approx. 20 minutes or until the center is just about set and only very slightly jiggly. Do not over bake! Cool to room temperature, then refrigerate for at least 2 hours to set. Dust with powdered sugar before serving.