Wash and scrub the unpeeled sweet potato, removing any undesirable areas. Pierce it 3 to 4 times with a knife and cook in a microwave for 2 to 3 minutes. Prick it with a fork. If it’s still firm, cook it for another minute until the potato is tender.
Once the potato is cool enough to handle, chop it into chunks. Place that in a food processor with the milk, vegan butter, and brown sugar. Pulse until smooth. Transfer this to a microwave-safe bowl (or use a saucepan over medium heat) and heat until it's warm to the touch (about 110°F/43°C).
In a large bowl, add the all-purpose flour, yeast, salt, cornstarch, and ground flax seeds. Stir to combine. Pour in the sweet potato mixture and stir to combine. Knead the dough in the bowl for 3 to 5 minutes. Add 1 to 2 tablespoons of all-purpose flour as necessary to keep the dough from sticking to the sides of the bowl. Spray the top of the dough with vegetable cooking spray, cover with a kitchen towel, and set it aside to rest for about 15 minutes.
Once the dough is ready, cover a working area with a sheet of plastic wrap. Place the dough on top of the plastic wrap and cover it with another sheet of plastic wrap. Use a rolling pin to roll the dough out to approximately 14x9 inches in size. It helps if you pull the top layer of plastic wrap off the dough and cover it again with the same sheet. It also helps to place an anti-slip pad to keep the dough from sliding as you roll it.
Spray a 9-inch round cake pan with nonstick cooking spray. Place rolls in the prepared pan. Cover with a kitchen towel, and set aside to rise in a warm, draft-free area until doubled in size, about one hour.