Preheat oven to 350°F and line a 9x13-inch baking dish with parchment paper.
Add the flour and powdered sugar to the bowl of a food processor. Pulse to mix then add the butter and pulse until combined and the dough starts to form a ball.
Remove the dough from the food processor and press into the bottom of the prepared pan, spreading it out evenly and to the edges. Bake at 350°F for 18 to 20 minutes or until the edges are slightly golden.
To Make the Filling
Whisk together the eggs, granulated sugar, lemon zest, lemon juice, and flour until completely combined. Pour over the baked crust and bake at 350°F for 25 to 30 minutes or until the edges are set and middle is still slightly jiggly.
Cool completely on a wire rack then refrigerate at least 2 hours or overnight. Cut into 36 squares and serve at room temperature. Dust with additional powdered sugar before serving.
Notes
Notes• If you use a flour blend that doesn’t contain xanthan gum, add 1 teaspoon when making the crust and ¼ teaspoon when making the filling.• The weight measurement for the flour is based on Bob’s Red Mill 1:1 flour. Other flour blends will not weight the same cup for cup.• You can mix up the filling while the crust is baking. Do not cool the crust for long before you add the filling.• Store in an airtight container in the refrigerator up to 1 week.