Preheat oven to 350°F. Line a 9x13-inch baking pan with foil, extending the foil over the edges. Spray foil with nonstick cooking spray; set aside.

In a large bowl, combine 2 cups flour, 1/2 cup powdered sugar and cornstarch. Using a pastry blender, cut in butter until mixture resembles coarse crumbs. Press mixture into bottom of prepared pan. Bake for 15 to 20 minutes or until the edges are light brown.

Meanwhile, for filling, in a medium bowl whisk together eggs, granulated sugar, 3 tablespoons flour, lemon juice, milk and lemon zest. Pour filling over hot crust bake for 15 to 20 minutes or until center is set. Cool completely in pan on a wine rack. Cover and store in fridge. Just before serving, use foil to lift uncut bars out of pan. Cut into bars and sift additional powdered sugar over top.