For the base mix together 1 cup of softened butter, 1/2 cup white sugar and 2 cups plain flour. Press into an ungreased square pan (20×20). Bake for 15 to 20 minutes (until firm) in a 175 degree oven.
While that is baking whisk together 4 eggs, 1 1/2 cups white sugar, 1/4 cup plain flour and the juice of 2 lemons. Pour over baked crust. Bake for another 20 minutes. It will set upon cooling in oven tray. Dust with icing sugar to serve.