Boil potatoes. Peel and puree or mash while hot. Cool completely. Cream butter until light, then work in egg yolks, salt, onion, and parsley. Mix into mashed potatoes, beating until well blended. Beat egg whites until stiff, then gently fold into potato mixture. Grease souffle dish and sprinkle bread crumbs on bottom and sides. Pour in the potato mixture and bake at 375 degrees F for about 45 minutes, or until nicely browned and risen.