Preheat the oven to 375°. Poke the sweet potato all over with a fork and bake for about 45 minutes, until tender. Peel and mash the potato. Set aside 3/4 cup of the mashed potato and let cool completely; reserve the rest for another use. Raise the oven temperature to 425°.
In a food processor, pulse the flour with the brown sugar, baking powder, baking soda and salt. Add the butter and pulse until the mixture resembles coarse meal. Add the buttermilk and the 3/4 cup of mashed sweet potato and pulse just until the dough comes together.
Turn the dough out onto a heavily floured work surface and knead 2 or 3 times, until smooth; the dough will be soft. Roll out the dough 1/4 inch thick and cut out eight 4-inch rounds. Arrange the sweet potato biscuits on a parchment paperlined baking sheet. Bake for about 15 minutes, until golden brown. Serve hot.
The unbaked sweet potato biscuits can be frozen in a single layer on a baking sheet for up to 1 week.