Puree cooked sweet potato in blender with water or milk and sugar.
Pour sweet potato liquid into a bowl, sprinkle with yeast, and allow to dissolve.
Sift dry ingredients together.
Stir flour mixture into yeast mixture. Knead in 1 additional cup of flour as needed.
Melt butter or lard and knead into dough.
Let rise until double (60-90 min).
Roll dough into flat rectangle; starting from front edge, roll dough into log. Cut log into 16 equal segments. (See photos and instructions for more attractive ways to shape the buns here: wendyinkk.blogspot.sg/2009/08/steamed-sweet-potato-mantou.html and here: wendyinkk.blogspot.sg/2011/08/purple-sweet-potato-rose-mantou.html).
Place buns in steamer basket, on cheesecloth, about 1 inch apart.
Steam vigorously for 15-20 minutes.
(The buns can be cooked in the steamer basket of your rice cooker while the rice is cooking. They don't all fit at one time, so I freeze the the rest of the shaped, raw, buns on a flat baking sheet, then transfer to a freezer bag. I can pull them out a few at time to thaw and steam as needed.).