Scrub and clean the sweet potato and poke it with a fork or knife several times. Place it in the microwave for two minutes. Remove with oven mitts and cut into large chunks.
In a food processor combine the roughly chopped sweet potato, water, milk, and brown sugar. Pulse until creamy. You might have small bits of sweet potatoes, but overall it should be creamy.
Transfer to a bowl and let the mixture cool slightly (it should be the same temperature as your finger when you dip it into the mix).
When the temperature is right, add the yeast. Gently stir. Set aside.
In a separate bowl combine 3 cups all-purpose flour, salt, and ground flax seeds. Whisk to combine.
Create a well in the middle of the flour and scoop the sweet potato mixture and 3 tablespoons melted vegan butter into the well. Stir to incorporate. Knead the dough in the bowl (3 - 5 minutes). Add up to 1/2 cup additional all-purpose flour as needed until the dough pulls slightly away from the sides of the bowl. It will still be sticky.
Spray the dough ball in the bowl with vegetable spray. Cover with plastic wrap or a kitchen towel and place in a warm, dry place to allow the dough to rise until it's doubled in size, roughly an hour or more.
When the dough has risen, prepare a 9X13" baking dish by adding a tablespoon of vegetable oil. Spread the oil around the pan.
The dough will be sticky so sprinkle whole wheat pastry flour, over the top and gently work the flour into the dough. The dough should reach the point where it's still sticky, but will form into a ball.
Next, pinch off sections of the dough a little larger than a golf ball. Dip each ball in the oil from the pan and gently pull the sides down, working the dough until it's the shape of a ball. It can help you work the dough if you occasionally touch your fingers into the oil in the pan. Place the dough balls about 1" apart on your prepared pan.
Spray a sheet of plastic wrap with vegetable spray and place that over the rolls (so the plastic won't stick to the rising rolls). Cover and let rise 30-40 minutes until doubled in size.
When you're ready to bake, heat your oven to 350F. When the oven is at the correct temperature, remove the plastic wrap from the rolls, and place the pan in the oven. Bake for 25 minutes, until golden brown on top.
After you remove them from the oven, immediately coat with a thin layer of melted vegan butter. These rolls are best when served warm, but can be made ahead and heated up prior to serving.
You can cover the dough ball in plastic wrap and either freeze or refrigerate. In addition, you can freeze dough balls (before they have risen) by putting them on a tray and placing them in the freezer, once frozen, transfer to a freezer bag.If you're working with refrigerated or frozen dough, be sure to allow additional rising time in the subsequent step.