Peel and slice potatoes. Cook in salted water until well cooked and soft, about 20 minutes. Pour in colander and drain well. Place on a baking sheet and put in oven at 350° F for about 5 minutes to dry. Remove from oven.
Place hot potatoes in a potato ricer and process in a large bowl. Add butter and mix well with wooden spoon until the butter is melted and well blended (do not use electric mixer). Add salt and freshly ground pepper to taste. Add egg yolks, one at a time; mix well with a wooden spoon until well blended. While still warm, place mixture in a pastry bag with a 1M tip. Pipe onto baking sheets lined with parchment paper or Release foil. (Note: May be made in advance up to this point and frozen.)
Preheat oven to 400° F. Brush potatoes lightly with egg wash. Bake for 30 minutes or until lightly browned. Yield 5 to 6 serving.