In a large mixing bowl, add warm water, yeast, and sugar and let sit for 3 minutes. Add salt, melted butter, eggs, sweet potato and stir. Add flour one cup at a time until dough isn’t too sticky but all of the dough is incorporated. Knead with a electric stand mixer for 6 minutes or until the dough is smooth and has the consistency similar to chewed bubble gum. If kneading by hand knead for 10 minutes or until consistency is reached. Place dough in bowl and cover. Then let dough rise for 1:30 hours in a warm place. Gently punch dough down and form into 15 round balls. Place in a greased pan with enough room for the dough to rise. Cut a long slit in the middle of the roll with a knife. Mix 1 egg white with 2 tablespoons water and brush over the tops of rolls. Cover with plastic wrap and allow to rise for 1 hour. Preheat oven to 350 F. Bake for 15 minutes. Brush tops with butter and serve.