Make sure all of your ingredients are at room temperature or slightly warm. In a large bowl whisk together the mashed sweet potato, buttermilk, egg, butter, sugar, salt and yeast. Allow mixture to sit for 5 minutes.
Begin by stirring in the flour one cup at a time, mixing well after each addition. Continue stirring in flour until the dough becomes too stiff to continue stirring easily. Turn the dough out on to a floured surface and continue adding flour and begin kneading the dough until it becomes soft and smooth, kneading approximately 10 minutes total.
Form the dough into smooth ball. Transfer to a well greased bowl, cover the bowl with parchment paper that you've sprayed lightly with cooking spray (to keep the dough from sticking to the paper when it rises), cover with a kitchen towel, set in a warm spot and allow to rise until it has doubled in bulk (usually about an hour).
When dough has doubled, punch down with your fist and transfer to your lightly floured work surface. Cut the dough in half (I use a metal bench scraper), then cut each half into 8 - 10 equal pieces depending on the size of roll you want, setting the pieces aside on your surface. When all the pieces are cut, cover them with a towel and let them rest for 10 minutes before shaping.
Preheat oven to 375 degrees and make sure two oven racks are situated in the middle of the oven. Line two baking sheets with parchment paper.
Lightly flour your hands and shape each piece of dough into a ball by rolling between your palms. Make sure the tops of the rolls are smooth. Divide the rolls between the two baking sheets and cover with a towel. Let rolls rise for about 20 minutes, remove towel and bake in preheated oven for 20 - 25 minutes until golden brown.
Remove from oven and cool on wire rack. Store in plastic zip-lock type bags until ready to serve. To re-warm, wrap gently in foil and heat for about 8 - 10 minutes in a 350 degree oven.