In a medium bowl, combine the 2 cups flour, 1/2 cup white powdered sugar, 2 tbsp cornstarch, 1/4 tsp salt.
Using a fork, cut in butter into the mixture is blended and coarsely crumbled. Press into the bottom of an ungreased 9x13 inch pan.
Bake for 15 to 20 minutes in the preheated oven, or until firm and golden.
In another bowl, whisk together the remaining 1 1/2 cups sugar and 1/4 cup flour. Whisk in the eggs, lemon juice & lemon zest. Pour over the baked crust.
Bake for an additional 20 minutes in the preheated oven. Remove, cover the baking dish, and chill in refrigerator for 1 hour. Uncover and sprinkle with powdered sugar on the top. When you are ready to serve, cut into uniform 1 1/2 to 2 inch squares.