In a large bowl, dissolve yeast in warm milk. Add 1 teaspoon sugar; let stand for 5 minutes. Add the sweet potatoes, egg yolks, butter, salt, remaining sugar and 2 cups flour. Beat until smooth. Stir in enough remaining flour to form a soft dough.
Turn onto a floured surface; knead until smooth and elastic, about 6-8 minutes. Place in a bowl coated with cooking spray, turning once to coat top. Cover and let rise in a warm place until doubled, about 1-1/2 hours.
Punch dough down. Turn onto a lightly floured surface; divide into 30 balls. Roll each ball into a 10-in. rope; tie each rope into a loose knot. Place 2 in. apart on baking sheets coated with cooking spray. Cover and let rise until doubled, about 30 minutes.
In a small bowl, beat egg whites and cold water; brush over rolls. Sprinkle with sesame seeds. Bake at 350° for 15-17 minutes or until lightly browned. Remove from pans to cool on wire racks. Yield: 2-1/2 dozen.
Recipe by Bakepedia at https://www.bakepedia.com/sweet-potato-yeast-rolls/