Croquettes de Pommes de Terre Dauphine

These potato fritters are the high point of potato cookery, sitting atop a pyramid of advance preparation, none of it particularly complex except in overall conception. You start by making mashed potatoes, which you then beat together with egg yolk (and some white, too) to achieve pommes duchesse—Duchess potatoes, an appareil which, in the old cuisine, was piped through a pastry tube to form a scalloped circle around a roast or other platter-borne main course. Once you have pommes du