Preheat oven to 350 degrees F (177 degrees C) and place rack in center of oven.   Grease with butter (or use a cooking spray) a 8 x 8 inch (20 x 20 cm) pan.

FOR CRUST:    In the bowl of your electric mixer, or with a hand mixer,  cream the butter and sugar until light and fluffy.  Add the flour and salt and beat until the dough just comes together.  Press into the bottom of your greased pan and bake for about 20 minutes, or until lightly browned.  Remove from oven and place on a wire rack to cool while you make the filling.

FOR FILLING:  In your electric mixer, or with a hand mixer,  beat the sugar and eggs until nice and smooth.  Add the lemon juice and zest and stir to combine.  Fold in the flour.  Pour the filling over the shortbread crust and bake for about 20 minutes, or until the filling is set.  Remove from oven and place on a wire rack to cool. 

To serve: Cut into squares or triangles and dust with powdered sugar.   These are best eaten the day they are made but can be covered and stored in the refrigerator for a few days.

Yield: 16 - 2 inch (5 cm) squares or 32 triangles.