Take the potatoes. Wash and peel them and boil until only half done. Grate them, and take only the part that has passed through the grater, that it may be light. Then weigh out half a pound. Beat the yolks very light with the cream, mix with the potatoes and add the butter, onion, cayenne pepper, and salt. Butter a mould well, sprinkle it with dried and sifted bread crumbs, put the mixture in it, and set the mould in a pan of boiling water in the oven, cover the mould and bake half an hour. Turn out carefully on a platter, pour a cream or Hollandaise sauce around it, and garnish with parsley. Serve very hot with a cucumber salad with French dressing, as a fish course.