Work into two cupfuls of mashed potatoes pepper and salt to taste, a teaspoonful of flour, and the beaten yolk of an egg. Set aside until cold and stiff; take a tablespoonful in the hollow of your floured hand and shape it into a cup. Put into the centre a tablespoonful of minced ham, seasoned with pepper and mustard, enfold it with the potato, roll over and over until you have a round, smooth ball, dip into beaten egg, then into cracker-crumbs, and set in a cold place until stiff. Cook in deep boiling fat.