Cook the purple sweet potato:

Microwave:

Cup of Yum
1. Cut the sweet potato in half, place it into a microwave-safe bowl.
2. Add about ¼ C of water to the bowl and cover with a lid.
3. Microwave on high power for 5-6 minutes, until the sweet potato is soft and mashable. Mash with a fork until it's smooth.

OR Steam:

1. Place the sweet potato on a steaming rack over simmering water, cover with a lid and steam for 30 minutes, until softened. Mash with a fork until smooth.

OR Boil:

1. Cut the sweet potato into cubes.
2. Fill a large pot with water, bring to a boil, and cook the sweet potato for about 30 minutes, until soft. Again, mash with a fork until smooth.
3. If the mashed sweet potato is dry, add a little water to help moisten the mixture.
4. While the mashed sweet potato is still warm, add in the granulated sugar and oil/or butter and mix together until incorporated.

Make the dough:

1. Heat up the milk (in the microwave) until just lukewarm and add in the yeast. Give it a stir and leave it for a few minutes until it becomes foamy.
2. Next, to a stand mixer bowl fitted with a dough hook, add in the flour, egg, salt, mashed sweet potato, milk and yeast mixture. Knead until the dough comes together. Check to see if the dough is dry -- if it is, add in a little more milk (about a 1 teaspoon) at a time.
3. You may need to adjust for the moisture content in the mashed sweet potato and the amount of liquid to add to the dough. If the dough seems dry, add a little more liquid. Conversely, if the seems wet, add a little more flour until you attain a slightly tacky (not sticky) dough.
4. Continue kneading until the dough becomes smooth and elastic.
5. Place the dough into a lightly oiled bowl, cover and let it double in size in a warm location. As this dough is slightly heavier than regular dough, it may take a bit longer to double in size.
6. Once doubled in size, punch down the dough.
7. Divide and shape the dough into 12 even balls.
8. Place onto a parchment-lined baking sheet (I used a 9" by 13" baking sheet).
9. Cover and leave to proof until slightly puffy.
10. Towards to end of the proofing time, preheat oven to 375°F/191°C.
11. Bake for 14-16 minutes at 375°F/191°C, or until the buns reach an internal temperature of 190°F/88°C. Cover the tops of the buns with aluminum foil about halfway through the baking time to prevent excessive browning.
12. Remove from the oven and let cool on a cooling rack.
13. Store in an air-tight container for up to 3 days at room temperature. Or freeze for up to 3 months.