In a small bowl sprinkle the yeast over the water and add 2 tablespoons of the sugar. Stir to blend and start the yeast to dissolve.
In a large mixing bowl combine the remaining, sugar, salt, and 4 cups of the flour.
Blend in the shortening with your fingertips.
Add the yeast, sweet potatoes, eggs, and remaining flour. Turn the dough onto a floured board and knead until smooth and elastic. Place the dough in a greased bowl, turning to grease the top of dough. Cover with foil or plastic wrap and refrigerate.
Two hours prior to baking, remove the dough from the refrigerator and divide into 2 balls.
On a floured board roll the dough out to a large circle, about 1/4 inch thick. Gently spread with soft butter.
Cut into 16 pie shaped wedges. Roll up each piece, beginning with the large end. Tuck the points under and place on a baking sheet.
Let the rolls rise for one hour.
In a small cup, beat the egg white with a small fork till thick and foamy. Add water to blend. With a pastry brush, gently brush the tops of the rolls.
Bake at 400 degrees for 15 to 20 minutes. Serve hot with butter.