Cook potato, peel & mash. In a bowl, combine yeast with lukewarm water. Allow to stand for about 3 minutes until foamy; add butter, salt, sour cream & potato & mix well. Stir in flour, one cup at a time. When dough is completely blended, turn onto a lightly floured work surface. Knead dough about 10 minutes, until smooth & elastic. Place dough in a greased bowl, cover with plastic wrap & allow to rise in a warm draft free place until doubled in bulk, about 1 hour.