Line a 9×13-inch baking pan with parchment paper, leaving extra overhang on the sides for easy removal. Spray with nonstick cooking spray.
Click the link above to see how to make the shortbread crust.
While crust is baking, prepare the filling by whisking the eggs, granulated sugar, powdered sugar, zest, and salt together in a large bowl. Whisk in the lemon juice and orange juice, and then the flour, until combined. Remove the parbaked crust from the oven. Pour the lemon filling onto the warm crust.
Reduce oven temperature to 325ºF, and bake for 30 minutes, or until center is set. Place pan onto a wire rack to cool completely before covering and transferring to the refrigerator until chilled.
Using a sharp knife dipped in hot water and wiped dry, cut lemon bars into squares and dust with powdered sugar just before serving.