Adjust oven rack to middle position and preheat the oven to 400°F.
Combine sweet potato and 4 tablespoons melted butter in a large bowl. In a separate bowl combine flour, sugar, baking powder and salt and mix until combined. Pour dry ingredients over sweet potatoes and mix with a spoon until all the ingredients are incorporated.
Pour milk over sweet potato mixture and mix until a dough forms. Roll dough out into a 3/4 inch thick rectangle and using a 2 1/2 inch round cutter cut our biscuits. Brush each biscuit with remaining butter and place on a baking sheet. Place in the oven and bake until biscuits are just beginning to brown, about 20 minutes. Serve warm with soft butter.