Wash and scrub the sweet potato, removing any undesirable areas. Pierce the potato a couple of times with a knife and cook in a microwave for 2 minutes. Use your fork to test if it is done. If it’s still firm, bake for another minute until the potato is tender.
Once cool, chop it into large chunks and place them in a food processor along with the margarine. Pulse until smooth. It should result in approximately one cup of mashed sweet potato. Add the soy milk and pulse again until smooth. Set aside.
In a separate bowl combine the brown sugar, warm water and yeast. Let this set for approximately 10 minutes until the yeast has become frothy.
In a mixing bowl combine one cup of whole wheat flour, salt, corn starch, and ground flax seed. Stir until combined. To this mixture add the mashed sweet potato mixture and yeast mixture Beat on low speed 3 minutes, stopping occasionally to scrape down sides of bowl.
Add 2 ½ cups of the white, unbleached flour. Continue beating on low until flour is absorbed and dough is sticky, scraping down sides of the bowl as needed. You can determine if more flour is needed here. If the dough is too sticky, add flour by ¼ cupful until it begins to pull away from the sides of the bowl. Once the dough has reached the right consistency, place it on a lightly floured work surface and knead for about 8 minutes, adding more white unbleached flour as needed. Your goal is a smooth dough ball that bounces back when you gently press your thumb into it.
Spray a large bowl with nonstick cooking spray and place the dough ball into this bowl, turning it to ensure the top is coated with some of the oil as well. Cover bowl with a wet kitchen towel or plastic wrap sprayed with vegetable cooking spray. Place the dough in a warm area until the dough has doubled in volume, about 2 hours.