Preheat oven to 425˚F (220˚C). Line a large baking sheet with parchment or lightly grease.
In a large bowl, combine the flour, brown sugar, baking powder, and salt.
Quickly cut in the chilled and diced butter using a dough blender, two forks, or your fingers until blended and no pieces are larger than a pea.
Make a well in the center of the buttery mixture and add the sweet potato puree and buttermilk. Mix just until the dough comes together. If the mixture still won't come together, add a little more buttermilk a splash at a time. If the mixture is too wet, add a little more flour.
On a lightly floured surface, pat the dough into a rectangle about 3/4 inch (2 cm) thick, flouring the top only as needed to keep it from sticking to your hands. Use a 2 1/2 inch (6 cm) wide round cutter to cut out 8-10 round biscuits, being careful not to twist the cutter as you press down. Place the biscuits 1-2 inches (2.5-5 cm) apart on the prepared baking sheet. Gently press together the scraps and cut to form additional biscuits.
If desired, brush the tops of the biscuits with a little melted butter. Bake in preheated oven until flaky and bottoms are lightly golden brown, 13-15 minutes.
Serve warm from the oven.