Adjust the oven rack to the middle position. Preheat the oven to 350 degrees.
Spray an 8 x 8-inch glass baking pan with cooking spray. Place a piece of parchment paper in the bottom of the pan, allowing the excess parchment paper to hang over the sides, to aid in removing the baked lemon bars from the pan. Spray the parchment paper with cooking spray.
Prepare the crust by combining the flour, sugar, salt, and butter, and beat with a hand mixer on medium-low speed until the mixture resembles coarse wet sand. Don't overmix or let the mixture come together into a dough.
Firmly press the dough into the bottom of the pan. Place into the freezer for 15 minutes.
Remove from the freezer and immediately place into the oven.
Bake until light golden brown, about 20 to 25 minutes.
Remove the crust from the oven and set it aside.
Reduce oven temperature to 300 degrees.
Meanwhile, make the filling. In a large bowl, whisk together the sugar, flour, and cornstarch until well combined. Add the lemon juice and eggs and whisk until combined.
Strain the mixture directly onto the prepared crust.
Place in the oven and bake for 30 minutes, or until the filling no longer jiggles when tapped.
Cool on a wire rack for 15 minutes, then refrigerate for 2 hours to set.
Cut the lemon bars into 9-12 squares.
Sprinkle a bit of powdered sugar through a sieve over the top of the bars. Serve and enjoy.