1. Whisk together the flours, yeast, salt, spice, and sweet potato.
2. Stir in the milk and water until the dough forms ball.
3. Cover the bowl with plastic wrap or a damp towel and let rise in a warm room (around 70 degrees F) for 12-15 hours.
4. After the dough has risen, scrape it out of the bowl and onto a lightly floured surface. Fold it a couple of times, then shape into a round loaf. Place loaf on parchment paper and return to your bowl, again covering bowl with plastic wrap or a damp towel. Let rise until doubled in volume (1-2 hours). (Cooler temps need longer proof times.)
5. While the dough is almost done proofing, place a large, heavy-bottomed, oven-proof, lidded pan in oven and preheat to 400 degrees F (20 minutes).
6. When dough is done proofing and oven is hot, score the top of the loaf deeply with a very sharp knife. Transfer onto parchment paper, place in preheated pan, and cover. Bake until loaf reaches an internal temperature of 195 degrees F (50 minutes), keeping the oven closed during the full bake time.
7. Remove from the pan and let cool for at least 1 hour before cutting and serving, warm or cool.