Bake 2-3 sweet potatoes at 400 degrees for about an hour, until very soft. Place in the fridge to chill. (I often do this a day ahead, or save a few baked sweet potatoes from a meal earlier in the week.)
Place 1 1/2 sticks (12 tablespoons) of butter in the freezer and preheat the oven to 500 degrees F. Line two baking sheets with parchment papers and set aside.
In a large bowl, combine all the dry ingredients and stir to blend. Then peel and mash the sweet potatoes, and measure out 1 1/2 cups to add to the dry mixture. Keep cold until ready to use.
Using a cheese grater, shred the butter onto a piece of wax paper and put the butter back into the freezer until ready to use.
When ready to mix the wet and dry ingredients, WORK FAST, and keep your hands off the dough as much as possible... it must stay cold until it goes in the oven to create light flaky biscuits. Add the mashed sweet potato to the dry mix and stir to combine. Then add the buttermilk and frozen shaved butter and stir to combine.
Dump the dough out on a floured work surface. Use a floured rolling pin to roll the dough out into a 1-inch thick rectangle. Fold the dough in thirds. Roll again and repeat. Fold in thirds and roll out one more time. Add flour as needed. Use a 3 inch biscuit cutter (or a drinking glass) to cut out the sweet potato biscuits. Flour the glass as needed, and cut the biscuits very close together. Cut all 16 biscuits out the first time if possible. If not, gather and roll the dough and cut again.
Place the biscuits on the prepared baking sheets. Brush the tops with melted butter and place in the oven while they are still cold. Bake for 8-10 minutes until tall and golden. (If the dough is not cold, it's best to pop the baking sheets in the fridge for a few minutes before baking.