• Position an oven rack in the center of the oven. Heat the oven to 350F degrees. Line an 8-inch square baking pan with foil, allowing the foil to extend over the rim by 2 inches. Spray the foil with non-stick spray.

To make the crust:

• In a small bowl, whisk together 2/3 cup of the flour, cornmeal, baking powder and salt until well blended; then set it aside.
• In another small bowl, beat together the eggs and egg white.
• Take 1 tablespoon of the egg mixture and place it in a cup, along with the teaspoon of water, and stir it together. Set aside both egg mixtures.
• With an electric mixer on low speed, beat the butter in a medium bowl until creamy. Add 1/4 cup of the granulated sugar and beat until blended. Beat in the egg-water mixture. Add the flour mixture and beat until blended. (The dough will be crumbly.)
• Press the dough mixture evenly onto the bottom and 1/4-inch up the sides of the foil-lined pan. Bake until golden brown, 20 to 25 minutes.

Make the filling while the crust bakes:

• In a medium bowl, whisk together the remaining 1/3 cup flour and 3/4 cup granulated sugar. Whisk in the remaining egg mixture until blended. Add the lemon zest and lemon juice and whisk until well blended.
• Remove the crust from the oven and place on a rack. Reduce the oven temperature to 300F degrees. Pour the lemon mixture over the hot crust. Bake until the filling is set, 20 to 25 minutes.
• Remove from the oven and place on a wire rack to cool. Refrigerate until cold, at least 1 hour and up to 8 hours.
• When ready to serve, sprinkle the chilled bars with confectioners' sugar. Use the foil to lift the slab of bars from the pan. Cut into 16 even squares.