Preheat oven to 425.
Line a baking sheet with parchment paper.
In a small bowl, stir together the sweet potato and 1/4 cup buttermilk.
In a large bowl, whisk together the flour, sugar, baking powder, and salt.
Cut in the butter with a pastry blender, 2 forks, or your hands until the butter is the size of peas.
Add the sweet potato mixture, folding to combine.
Add the remaining buttermilk a little at a time until all the flour is moistened. (You may not need all of the buttermilk.)
Sprinkle work surface with flour.
Place the dough on the floured surface, and pat into a rectangle about 3/4" thick.
Fold it into third, and roll with a floured rolling pin until the dough is 3/4" thick again.
Cut into 10 rectangles.
Place the biscuits on the prepared baking sheet, and bake for 20 minutes or until light golden brown and firm to the touch.
Transfer to a baking rack.
They are best eaten warm!
