Mix flour and powdered sugar together (for crust).
Cut butter into flour mixture (as for pie dough).
Spray 9 x 13 baking dish with Pam nonstick spray.
Spread crust into bottom of pan and pat lightly.
Bake at 350° for 20 minutes.
Beat all of the topping ingredients together lightly.
Pour over the toasted crust and bake an additional 25 minutes at 350°.
Cut into small squares.
Be careful not to overcook or they get very crumbly. The crust should be slightly crumbly when you bite into it, but not fall apart in your hands. The topping should be the consistency of lemon meringue pie filling.