Adjust oven rack to lower-middle position and heat oven to 325 degrees. Spray an 8- by 8-inch baking pan with vegetable cooking spray. Fit an 8-inch by 16-inch piece of heavy-duty foil across pan bottom and up the 2 sides as foil overhangs to pull bars from the pan. Spray foil with vegetable cooking spray.
Mix flour, salt, confectioner's sugar in a medium bowl; stir in butter to form dough. Press dough into pan bottom (the bottom of a measuring cup dipped in flour helps). Bake until pale golden, about 20 minutes.
While pastry bakes, whisk eggs, sugar, flour, lemon juice and zest in a medium bowl.
Remove pan from oven. Add lemon mixture and continue to bake until just set, about 20 minutes longer; let cool in pan for a few minutes and then using foil handles, pull bars from pan and set on a wire rack. Cool until room temperature. Cut into squares and serve.