Boil potatoes in salted water, then peel and chop into coarse cubes. Brown the potatoes in an iron skillet in a generous amount of melted butter. When they are almost done, press them gently together against the bottom of the pan so that they form a cake. When the bottom is golden brown, carefully invert the pan over a serving plate so that the potatoes are served bottom-side up. Garnish with very finely chopped shallots or scallion.