Pre-heat the oven to 180 degrees C. Grease an oven proof serving dish and place in a roastinig tin.
1. Cream the butter with the lemon zest and sugar until pale and light. Add the flour, egg yolks, lemon juice and milk. Beat well.
2. Whisk the egg whites until stiff and glossy. Using a metal spoon, fold into the lemon mixture.
3. Pour into the prepared serving dish. Fill the baking tin with boiling water to reach about half way up the sides of the dish. Carefully move to the oven. Bake for about 60 minutes, the top will be a golden brown and may crack slightly.