Peel sweet potato and cut into cubes. Place cubes into a small sauce pan, add water until just covered, bring to a boil, reduce heat, and cook for 10-15 minutes until sweet potatoes are just tender. Remove from heat, cover with lid, and let sweet potatoes stand for 10 more minutes. Drain and mash, but leave a few small chunks of potato - they look so pretty in the finished product. Add buttermilk and set aside.
In a mixing bowl, whisk together white flour, wheat flour, baking powder, baking soda, salt, brown sugar, and black and cayenne peppers (if you're using pepper). Make sure the brown sugar is broken down and completely incorporated into the dry mixture.
Add the cold butter cubes. Cut the butter into the flour mixture using a pastry blender, two knives, or whatever you use to cut fat into flour. The mixture should look a little crumbly.
Add the sweet potato/buttermilk mixture to the dry ingredients. Stir together with a fork until all ingredients begin to come together.
Dump the flour/butter/sweet potato dough onto a floured surface. Gently knead the dough 15 times or so, until you have a nicely incorporated dough. The dough will be sticky, but resist the temptation to add more flour.
Gently pat the dough into an 8" circle, one inch high. Dip a 2" biscuit cutter in flour, and begin to cut the rounds. Bring scraps together to cut more rounds until all the dough is used. Place the rounds on a baking sheet lined with parchment paper or a silpat.
Bake the biscuits at 425 degrees for 15-17 minutes, or until they're nicely browned on top. Remove from the oven. Put the biscuits on a wire rack to cool. Serve warm.
Notes
You can use all white flour, if you don't have whole wheat flour on hand.