1. Preheat oven to 350º
2. Lightly grease an 9x9" square baking dish with butter
3. For the base, process the flour and confectioners sugar in a food processor for several seconds. Add the butter and process until the dough comes together, 20-30 seconds
4. Press the dough gently and evenly over the bottom of the prepared pan. Glaze it with the egg white: Pour the egg white on the dough and tip the pan from side to side so that the egg white spreads over the surface. Pour out the excess
5. Bake the base on the center oven rack until golden, about 20-25 minutes. Place the base in the refrigerator for 15 minutes to cool completely. Keep the oven on
6. Meanwhile prepare the topping: Gently whisk the eggs in a medium-size bowl until lightly mixed. Add the remaining topping ingredients and continue to whisk gently until they are blended
7. Pour the topping evenly over the base. Bake until the top is set and golden, about 20-25 minutes.  Allow the lemon squares to cool completely
8. Sprinkle the surface with confectioners sugar and cut into squares with a sharp knife that is dipped in hot water and wiped dry before each cut

Diane's Tip: If you plan to keep the squares longer than a day, put them in an air-tight container and refrigerate.  You can also freeze them