Preheat oven to 4500F. Line a baking sheet with parchment paper.
Sift the flour, baking powder, baking soda, salt, nutmeg, and cinnamon into a large mixing bowl; set aside. In a medium bowl, mix the mashed sweet potatoes with the sorghum and buttermilk until smooth and creamy; set aside.
With a pastry cutter or two knives, cut the butter into the flour mixture until it's fully incorporated and has a coarse, mealy consistency.
Make a well in the center of the flour-butter mixture. Pour the sweet potato mixture into the well. With a rubber spatula, begin folding it into the dry ingredients. Mix just to the point that a dough forms. Do not overmix.
Turn out the dough onto a floured surface and roll out to 1-inch thick. Dust a 3-inch round biscuit cutter with flour (or a 1 1/2- or 2-inch cutter for cocktail-size biscuits), and cut out the biscuits. Transfer them to the prepared baking sheet, spacing them about 1 inch apart. Brush the tops of the biscuits with buttermilk. Bake for 14 to 18 minutes, until golden brown. Serve the biscuits hot.