Cream butter and beat in egg yolks. Stir in breadcrumbs, potatoes and salt and pepper. Mix thoroughly. Knead and shape into a long roll. Cut off pieces about the size of a walnut. Flatten each piece in the palm of your hand and place a cube of ham in the middle. Shape into a round dumpling, enclosing meat completely. Cook, uncovered, in simmering soup or stock for 10 minutes. Cover and cook a further 5 minutes. Can be eaten with the soup, or drained and served as a side.