Preheat oven to 425ºF. Whisk together the flour, sugar, baking powder, baking soda, and salt.
In a separate bowl, combine the sweet potato and buttermilk. Add the buttermilk mixture to the flour. Add butter and either use a pastry knife or your hands to combine until butter reduces to pea-sized chunks.
Turn the dough out onto a floured surface. Lightly knead the dough about 6 times. Cut out the biscuits, gather scraps and repeat.
Arrange biscuits on a pan or pizza stone, lined with parchment paper, and brush tops with melted butter. Bake for 20 - 25 minutes.