Preheat oven to 425° and grease a baking sheet with cooking spray. In a large bowl, combine flour, baking powder, sugar, and salt. Add butter and use your hands to cut butter into flour until about pea-sized. Add mashed sweet potato and buttermilk and stir until a rough dough starts to come together. Use your hands to help knead flour into biscuits until it just holds together.
Dump dough onto a clean surface and bring dough together, using your hands to help incorporate any dry flour bits.
Pat dough into a ½” thick square. Fold dough in half from left to right, then fold in half again from top to bottom. Pat dough back out into a ¾” thick square.
Use a 2 ½” round cutter to cut biscuits out and place on prepared baking sheet. Gather scraps together and pat back down into ¾” thick square one more time to cut a few biscuits out.
Brush tops of biscuits with melted butter and bake until tops and bottoms are golden, about 20 minutes.