Boil the potatoes with the skins on, allow them to cool enough, so you're able to handle them, then peel them. Mash them, then knead with the flour, semolina flour, salt, nutmeg, butter and egg yolk until you achieve a smooth dough that's slightly sticky.
500 grams potatoes, 50 grams butter, 100 grams all-purpose flour, 50 grams semolina flour, 1 egg yolk, 1 pinch nutmeg, ½ teaspoon salt