Well butter a souffle mould; failing that, use either a deep au gratin dish or a pie-dish. Melt the butter in a saucepan, add the mashed potatoes and beat them together with a fork until they are hot and light. Add the chopped fish, parsley, and milk. Separate the yolks and whites of the eggs. Beat up the yolks and stir them into the mixture; season it carefully, and lastly stir in the stiffly whipped whites of the eggs. Turn the mixture into the mould, or dish, and bake it in a quick oven until it is well puffed up and nicely browned. It will take about half an hour. Serve it at once.