Preheat the oven to 350 degrees.
Spray a nine by thirteen baking pan with cooking spray.
If you prefer, you can line the pan with parchment paper or aluminum foil to aid in removing the bars from the pan. Spray the parchment or foil with cooking spray.
In this recipe, you start by creating the shortbread base for the lemon bars.
Using a fork, stir together in a medium bowl the flour, cornstarch, salt, and the sugar.
Cut the stick of butter into small pieces to help you more easily incorporate it with the dry ingredients.
Using a pastry blender or a fork, cut the butter into the flour mixture. Take your time and be patient.
By the time the butter is fully incorporated there should be no large chunks of butter and the mixture should resemble cornmeal with small blobs in the mixture.
Using the back of a measuring cup, firmly press the dry mixture in the prepared pan. This should create an even layer all over the base of the pan.
Bake this layer in the oven for about 20 minutes, or until the top edges are just barely beginning to brown.
Remove it from the oven, but leave the oven on.
While the crust is baking you can prepare the lemon custard.
To cut down on dirtying more dishes, you can use the same bowl for preparing the custard layer that you did for the crust.
Crack the eggs into the bowl and whisk them well.
Add the sugar and lemon juice along with the flour.
Whisk for a few minutes to fully incorporate all of the ingredients.
Using a rubber spatula to scrape the custard out of the bowl, pour the lemon mixture over top of the pre-baked crust. You don’t have to allow the crust to cool.
Put the lemon bars back into the oven and bake for about 20 minutes or until the center of the lemon bars is set.
Remove the lemon bars and allow them to cool to room temperature.
Shake the powdered sugar over the top of the lemon bars before slicing them into squares and serving.
To store the lemon bars, cover the pan tightly with foil or place the bars into an airtight container. Store any leftovers in the refrigerator.
o freeze the lemon bars, you can place them into a freezer bag, press out as much air as you can, and freeze them for up to three months.
To thaw, place the frozen bars on a plate in a single layer and allow them to thaw in the refrigerator overnight, lightly covered with plastic wrap.