Cream together the butter and sugar, than add the flour. Mix well then add the lemon juice and the zest. Add the milk and the beaten egg yolks.
In another bowl, beat the egg whites until stiff. Stir into the lemon mixture. Mix well then pour into pie dish. Stand dish in hot water then bake in preheated oven until golden brown, about 20 minutes.
When cooked, top will be firm but bottom will have the consistency of custard.