|Peel potatoes, cut into quarters, and boil until done (tender).|
Drain off water and let dry (if possible in open warm oven).
Mash well, add butter, salt and pepper mixing well over a low fire, stirring until smooth. A double boiler is recommended.
Remove from fire, add egg yolk and mix well for about five minutes (if possible in warm water to retain heat).
Serves 4 to 6.