Start by making the shortbread crust. In a bowl, combine the powdered / icing sugar, white granulated sugar and flour.
Using your fingertips, rub the butter into the sugar and flour mixture and mix to form a soft dough.
Press out the dough evenly into a greased and lined 8 inch / 20cm square container and prick the crust with a fork. This will let the steam out ensure that the crust does not rise. Bake at
320°F / 160°C for 15 minutes until lightly browned. Remove from the oven and pour the lemon filling over the base.
TO MAKE THE LEMON TOPPING
Using an electric, handheld beater or whisk beat the eggs and sugar very well, for about 5 minutes. This will help dissolve the sugar and avoid the grainy texture once the lemon squares have been baked.
Add in the lemon juice, lemon zest and flour and beat for 1 minute or until smooth. Pour this filling over the partially baked crust. Return it immediately to the oven and bake for a further 20 to 25 minutes until golden brown and firmly set.
Allow to cool before slicing and dusting with powdered / icing sugar. SEE NOTE 1
It is important to allow the lemon bars to cool completely before slicing. This ensures that the inside is well set and makes slicing into neat squares easier. To help speed the cooling process, I usually pop the tin into the refrigerator for about 30 minutes or until completely cool before slicing.