Preheat oven to 425°F. Line a large baking sheet with parchment.
Whisk self-rising flour, sugar, baking powder and salt in a mixing bowl. Cut in shortening until mixture resembles sand and pebbles. Pat sweet potatoes dry and chop; toss with flour mixture. Gently mix in buttermilk.
Knead dough gently 3 or 4 times on a lightly floured counter. Roll out to 3/4-inch thickness. Cut with a 2 1/2 -inch biscuit cutter. Pat together dough scraps and cut out more biscuits. Bake until tops are golden brown, 14 to 18 minutes. Brush with butter and serve hot.