Place the egg yolks in glass dish and fold in the castor sugar, mix until the texture becomes uniform. Add the juice of all the lemons and grated rind of one lemon, mixing with care.
Take a large saucepan big enough to hold the glass dish, to form a bain marie, fill half way with water and bring to boil. Gently lower the glass dish with the mixture and cook in the bain marie for 8 minutes at least. Stir regularly and and when finished set aside to cool.
Whisk the egg whites adding a pinch of salt and the icing sugar. Fold into the cooled mix stirring with a spoon from the bottom to the top. Spoon the finished mixture into 4 glasses and chill in the fridge for 4 hours.