Cook scrubbed potatoes until tender. Drain and peel. Return to pan and shake over low heat until dry. Mash very smooth and mix in egg yolks, salt, and butter. Gradually add flour and work into a smooth, manageable dough (should hold shape and be pliable, but not sticky). Break into several sections. Flour hands and board, then roll dough between palms, and board until you have ropes about the thickness of your thumb. Cut into 3/4-inch lengths. Poach at a slow boil in salted water. Do not crowd. Remove with a slotted spoon to drain and place in baking dish with a tomato sauce, (or a tomato and meat sauce). Top with grated cheese if desired. Heat in a 325-degree oven for about 30 minutes. This dish may be prepared in advance and heated before serving.