Preheat oven to 350°F. Line a 13 x 9 inch baking pan with parchment paper, leaving an overhang to form handles for lifting from the pan. Spray parchment with cooking spray or lightly butter.
In a medium size mixing bowl, use a whisk to sift together flour, sugar, cornstarch and lemon zest. Using a pastry blender cut the cold butter into the mixture until it resembles peas. Press evenly into the prepared pan.
Bake crust for 20 minutes just until lightly golden around the edge.
Meanwhile prepare the filling: In the same bowl, whisk together sugar, lemon juice, eggs, flour, lemon zest and salt until fully blended. Pour over warm crust.
Place back into the oven and bake for 25-35 minutes OR until the center is set when gently shaken.
Cool completely in the pan on a wire rack. Lift from the pan using the parchment paper.
Use a sharp knife to cut into 20 even squares or desired size pieces. Dust with powdered sugar and serve.
Store leftovers in an airtight container, chilled for 3-5 days.