In a small bowl, add the egg and sweet potato. Mash and mix with a fork.
Mix with your hand the sugar and butter into the sponge. Then mix in the all but 1/2 cup of the flour.
Once the dough comes together, dust a work surface and knead dough for about 20 minutes, working the remaining flour into the dough.
Place dough in a greased large mixing bowl. Turn the dough to light coat and cover loosely with plastic wrap. Set aside in a warm area of the kitchen for 1 hour.
Punch down dough. Knead back into a ball, then portion it into small (40g) balls. Once weighed, take two balls, one in each hand and roll them into smooth round balls. This is done by applying a little pressure and rolling them in a circular motion on your work surface.
Place dough balls in a greased pan or cast iron skillet. Set about half an inch apart. Cover and let double in size.