Preheat oven to 350 degrees F and line a 9x13 baking dish with foil, making sure to leave enough foil to overhang on the sides to pull the dessert out when cooled. Coat with cooking spray.
In a large bowl, whisk together 2 cups of flour, xanthan gum if using gluten-free flour, confectioners' sugar, lemon zest, and salt.
Add the butter to the flour. Using your hands, work the butter into the flour mixture, until clumps form. Mixture will become crumbly.
Press into the 9x13 pan, trying to distribute as evenly as possible.
Bake for 20 to 25 minutes, until edges slightly brown. Allow to cool 5 minutes.
While the crust is baking, whisk together the eggs, granulated sugar, lemon juice, and 1/3 cup of flour. Whisk until the mixture is smooth and no clumps are visible.
Pour mixture over crust and bake 20 to 25 minutes, until bars are set.
Place pan on a cooling rack to cool completely.
Once completely cool, lift foil to remove bars from pan. Peel the foil away from the sides.
Dust with confectioners' sugar and slice into bars. Store in refrigerator. Enjoy!
