Boil potatoes in salted water until tender, about 12 minutes. Drain potatoes and pour them into a bowl. Combine sour cream, egg yolk and cream. Add the cream mixture into potatoes and mash until potatoes are almost smooth. While potatoes boil, preheat a large skillet over medium high heat.
http://www.foodnetwork.com/recipes/rachael-ray/30-minute-shepherds-pie-recipe.html