In a large bowl, sift together the flour, baking powder, baking soda and salt.
Cut the butter into the dry ingredients with a pastry cutter until the mixture resembles a coarse meal with a few pea-sized pieces of butter remaining.
In a large measuring cup, whisk together the sweet potato and the buttermilk. Pour into the dry ingredients and stir to combine, just until barely combined.
Turn the dough out onto a floured surface. Dough may be sticky; if so, lightly flour the top of the dough. Use your hands to bring the dough into a ball, then gently pat into a disk. Fold dough in half, turn it 90 degrees, and pat out again, flouring the dough as needed. Fold in half again. Repeat this process 1 more time. After the last fold, roll dough to 1-inch thick. Cut with a biscuit cutter; press cutter straight down, then twist to remove the biscuit. Re-roll dough as necessary, noting that rerolled biscuits will not get quite as much "lift" as the first batch. Cut 8-9 biscuits.
Place cut biscuits in a 10-inch ungreased cast iron skillet, or close together on an ungreased baking sheet. Bake for 12-14 minutes, or until golden on top.