Place a rack in the center of the oven and preheat to 425 degrees F. Grease a baking sheet (with butter, oil or cooking spray).
In a small bowl, whisk together the sweet potato and 1/3 cup milk. Set aside.
In a large bowl, whisk together the flour, sugar, baking powder and salt. Cut in the butter with your hands, a pastry blender or two knives until the mixture resembles coarse meal. Add the sweet potato mixture and fold gently to combine. Add the remaining milk a little at a time until all the flour is moistened. The amount of milk you will need will depend on the moisture of the sweet potato.
Sprinkle a small handful of flour on a work surface. Turn the dough out onto the surface and knead lightly 2 to 3 times with the palm of your hand until the mixture comes together. Pat the dough out into a 1/2-inch-thick round.
Using a 2 1/2-inch-round biscuit cutter, cut the dough into biscuits. Gently reroll the scraps and cut out more biscuits. Place the biscuits on the prepared baking sheet and bake until light golden brown and firm to the touch, 12 to 14 minutes. Serve these fluffy biscuits warm or at room temperature.
