1. Preheat a fan-assisted oven to 180 degrees Celsius, or equivalent. Lightly grease a 20cm x 20cm deep-sided square (or rectangular) baking tin and line with parchment paper ensuring the paper comes up slightly higher than the rim of the tin (to avoid leakage as the filling is added).
2. For the shortbread base: In a large wide mixing bowl rub the shortbread ingredients (plain flour, butter and icing sugar) together using your fingertips until they resemble breadcrumbs , alternatively, blitz the ingredients in a food processor.
3. Press the shortbread mixture into the lined tin using your fingertips, then smooth and level the surface using the back of a spoon. Bake in the preheated oven for 20 minutes or until the shortbread is pale golden in colour. Remove from the oven and set aside in the tin.
4. For the lemon filling: Using an electric whisk, beat together the eggs and caster sugar until the mixture is light and airy.
5. Stir in the lemon juice and lemon zest.
6. In a separate bowl, sieve together the flour and baking powder, then whisk or fold them into the egg mixture.
7. Pour the lemon filling over the baked shortbread.
8. Bake in the preheated oven for 17-20 minutes. Remove from the oven once the top has turned golden and the filling still has a slight wobble in the very centre (it will continue to set as it cools). Leave to cool fully in the tin. Store it, wrapped air tight, in the refrigerator.
9. When ready to serve cut into squares and serve dusted with a little extra icing sugar.
Variation: For a hint of orange flavour, substitute half orange juice for lemon juice. Alternatively add some orange zest to the shortbread.