Ingredients:
Preheat oven to 180ºC (350ºF).
Grease an 8x8 baking dish, and a bain marie for baking.
Cream together the butter and ¾ of the sugar until light and fluffy.
Beat in the egg yolks, and the lemon zest.
Mix in the flour, salt, milk and lemon juice.
Whip the reserved egg whites, slowly adding the reserved sugar, until smooth and glossy.
Vigorously stir ⅓ of the whipped egg whites into the butter flour batter.
Slowly fold in the remaining egg whites.
Transfer the batter to the baking dish and fill the bain marie with hot water.
Bake for 30 -35 minutes.
