Preheat oven to 350. Spray a 9x9 pan with cooking spray. Combine flour and sugar in mixing bowl. Add butter mixing until coarsely crumbled. Press into pan and bake for 17 min. Cool. Zest lemons to get 2 tsp of zest; finely chop. Juice lemons to measure 1/2 cup juice. In a small mix bowl, whisk zest, juice and sugar. Add eggs, egg whites, flour and salt; whisk until blended. Evenly pour filling over cooled crust. Bake 20 min or until set. Cool on cooking rack then sprinkle with powdered sugar. Cut into bars.