Preheat oven to 425 degrees F. In a large bowl, mix together flour, sugar, baking powder, salt, and baking soda. Cut in butter with a pastry blender until mixture resembles coarse crumbs. In a small bowl, mix together sweet potato puree and buttermilk. Stir into flour mixture until combined, and do not overmix.
Turn out dough onto a lightly floured surface, and knead lightly until dough comes together, but is still slightly lumpy. Shape into a 1" thick disk. Cut out biscuits 2" in diameter. Gather together scraps, press into a 1" thick disk, and cut out more biscuits, 9 total.
Grease an 8" square pan. Place biscuits in pan (biscuits will be touching). Brush with melted butter. Bake until golden, 20 to 24 minutes.