For the dough, bring 2 cups water to a boil in a medium saucepan. Add potatoes and cook, covered, over moderate heat until tender when pierced with a fork. Drain, reserving 1 cup of the water. Mash potatoes and measure 2/3 cup. In a large bowl with an electric mixer, mix together the hot potatoes, sugar, butter or margarine, salt, lukewarm potato water, egg, egg yolk and yeast. Add the flour and mix on low speed until incorporated. Mix on medium speed for 1 minute; the dough will be sticky.