1. Preheat the oven to 375 degrees F. Line a baking sheet with parchment paper.
2. Prick sweet potato all over with a fork. Place the sweet potato and 1/4 cup water in a microwave safe dish. Cover with plastic wrap. Microwave for 6 to 7 minutes or until fork tender. Let cool. Peel sweet potato; discard skins. Mash sweet potato flesh with a fork or potato masher (you should have about 1 cup).
3. In a medium bowl whisk together the flour, sugar, baking powder, salt, cinnamon, and nutmeg. Using a pastry blender, cut in shortening and butter until mixture resembles coarse crumbs. Stir in the potato and buttermilk just until mixture begins to come together.
4. Turn dough out onto a floured surface. Knead dough by folding and gently pressing it just until dough holds together. Roll or pat dough into an 10x5-inch rectangle, about 3/4-inch thick. Cut the dough into 12 smaller rectangles, then carefully transfer them to the prepared baking sheet. Bake until the bottoms of the biscuits are golden, about 25 minutes. The biscuits will be quite soft, but they will firm up as they cool.