Beat the egg yolks and the sugar until thick and creamy. Stir in the butter, lemon rind, lemon juice, milk, fold in the sifted flour. Beat the egg whites until soft peaks form, add the extra sugar in batches, beating until dissolved between each addition. Fold the stiffly beaten whites into the lemon mixture in 2 batches.
Pour into a lightly greased casserole dish and stand the casserole in a large baking dish with sufficient hot water to come halfway up the side of the casserole. Bake at 350° for about 50 minutes or until the top of the pudding is set (it separates out into a light airy sponge topping with a lemony sauce underneath). Dust with a little sifted icing sugar and serve as soon as possible, while still warm (it may sink a little but this is just because it is so light).