Adjust an oven rack to the middle position and preheat the oven to 350°F.
Zest lemon and save 1 Tbs.
In a food processor, blend together flour, cornstarch, salt and sugar and any leftover zest. stop, add cubed butter and process until it starts forming a dough.
Press into the bottom of parchment lined 8x8 inch pan.

Bake for 15 to 20 minutes in the preheated oven, or until golden.

Whisk together the remaining sugar and flour. Whisk in the eggs, lemon juice, salt and zest. Pour over the baked crust. Bake for an additional 20 minutes.

The bars will firm up as they cool.

Sprinkle powdered sugar over before serving.