n a large bowl, whisk together the flours, salt, baking powder, sugar and baking soda.

Using a grater, a pastry cutter or your hands, cut the butter and vegetable shortening into the flour.  The flour should have pea sized flecks of butter.  Cover the bowl with plastic wrap and place in the freezer for 30 minutes. 

Remove the bowl from the freezer and create a well in the flour add in the sweet potato

Slowly pour in 1 ½ cups of buttermilk and mix with the spatula until a shaggy dough begins to form.  Stir  in the remaining buttermilk and gently begin to bring the dough together with your hands in the bowl before turning the dough out onto a floured countertop. If the dough doesn’t come together or looks dry, add a tablespoon of buttermilk at a time until the dough forms. 

Pat the dough into a 1 inch  thick rectangle and complete a letter fold– fold the right side of the dough to the middle and fold the left side of the dough over the right.  Rotate the dough 90 degrees and repeat 2 or 3 more times.

Pat out the dough to ½  inch thick rectangle. 

Flour a 2-inch biscuit cutter and cut out the biscuits, without twisting the cutter. 

Place the cut out biscuits on a parchment lined baking sheet with 1-inch between each biscuit.  Place the whole cookie sheet in the refrigerator while you preheat the oven.

 Preheat the oven to 375ºF 

Bake the biscuits for 25-30 minutes.