Arrange a rack in the middle of the oven and heat to 325 degrees F. Lightly coat an 11x7-inch glass baking dish with cooking spray or butter. Line the dish with a parchment paper sling, leaving an overhang of about 2 inches on each long side; set aside.
Make the Crust:
Place the flour, granulated sugar, powdered sugar, zest, and salt in the bowl of a food processor fitted with the blade attachment. Pulse 5 times to combine. Add the butter and pulse until the mixture looks like coarse crumbs.
Press the Crust in and Chill:
Pour the crust mixture into the prepared baking dish. Use the bottom of a measuring cup or your fingers to press the crumbs into an even layer. Refrigerate for 15 minutes.
Prebake the Crust:
Bake the crust until it is a light golden brown, 25 to 30 minutes.
Make the Filling:
Whisk the whole eggs, egg yolk, granulated sugar, powdered sugar, zest, and salt together in a large bowl. Whisk in the lemon juice, and then the flour, until combined.
Pour the Filling into the Crust:
Remove the crust from the oven and pour the lemon filling onto the warm crust.
Bake the Lemon Bar:
Return the pan to the oven and bake until light brown around the edges and the top appears relatively dry, 15-20 minutes. Cool completely on a wire rack before cutting.
Dust with Powdered Sugar:
Remove the lemon bars from the pan using the parchment sling. Use a fine-mesh strainer to generously dust the bars with powdered sugar.
Cut the Lemon Bars:
Place on a cutting board and cut into 12 pieces. Dust with more powdered sugar, if desired, before serving.
