Heat oven to 450°F (232°C). In the bowl of a food processor, add the flour, baking soda, salt and sugar, pulsing once or twice to combine well. Add the butter or shortening, and sweet potato and pulse for approximately 15 seconds until the mixture resembles coarse meal.  Pour in the buttermilk a little at a time between pulses, just enough to form a cohesive dough that is not too soft. (If you overdo the buttermilk, just try to knead in a little extra flour in the next step.) 

On a floured surface, knead the dough about 10 times. Roll out to 1/2 inch (1.25cm) thickness, and cut with a biscuit cutter or a glass, about 2 1/2 inches (6.35cm) in diameter. Bake on an ungreased cookie sheet for 10 – 12 minutes.