Pre-heat the oven to gas mark 4,350 F (180 C)

first beat the butter, sugar and lemon rind together until well softened.

Then beat in the egg yolks, a little at a time.

next fold in the flour, alternately with the milk and lemon jucie.

Finally whisk the egg whites and fold them in as well.

Pour the mixture into the baking dish and bake in the center of the oven for 45-60 minutes.