For sweet potato puree, pierce one large (12-ounce) sweet potato in a few spots with a fork and microwave at high power for about 10 minutes or roast at 350° for about 1 hour, until tender. Let the sweet potato cool slightly, then peel and puree in a food processor. Chill.

Preheat the oven to 400°. In a small bowl combine the sweet potato puree and the buttermilk. Whisk to combine, set aside.

In the bowl of a food processor, combine the flour, baking powder, salt, and sugar. Pulse several times to combine well. Add the butter, and continue to pulse in 2-second increments until the mixture resembles coarse crumbs with a few pea-sized pieces (about 5 pulses). This can also be done using a pastry blender or 2 forks if you don’t have a food processor. Transfer the mixture to a medium bowl, pour the sweet potato mixture over it, and use a fork to mix for about 1 minute, or until the dough just comes together.

Turn the dough out onto a floured surface, knead with floured hands, and pat into a rectangle about 6×10 inches and 1-inch thick. Fold the dough like a business letter (the rightmost third over the center third, then the left third on top). Turn the dough a quarter of a turn, pat it into another 6×10-inch rectangle, and fold it upon itself in thirds again. Repeat one more time, then pat the dough into a 6×10-inch rectangle a final time.

Using a floured 2-inch biscuit cutter, cut the biscuits from the dough and place them about 1½ inches apart on an ungreased baking sheet. Pat the scraps into a 1-inch thick rectangle and cut more biscuits. Brush the tops with half the melted butter, and bake 15-20 minutes, or until the tops are just beginning to brown. Brush the tops again with the remaining butter and serve warm.