Preheat oven to 350 degrees and line the bottom of a 9" x 13" baking dish with parchment paper. Spray parchment with cooking spray and set aside.
To make the crust, blend together flour, sugar and salt in a medium bowl until well combined, then add butter and mix until smooth dough forms. Press dough evenly into bottom of baking dish and bake 20–22 minutes until just set and pale.
Remove from oven and set crust aside. Reduce oven temperature to 300 degrees.
To make the filling, stir together sugar and flour until no lumps of remain. Add lemon juice and lemon zest and stir until smooth. Add beaten eggs and whisk until completely combined, then pour filling into crust.
Bake at 300 degrees for 25–30 minutes or until the filling is set in center. Cool completely on a wire rack, then cover and refrigerate until completely chilled.
Once completely chilled, cut into squares. Dust with powdered sugar, if desired. Refrigerate any leftovers for up to 5 days.