Cook the purple sweet potato:

Microwave:

• Cut the sweet potato in half, place it into a microwave-safe bowl.
• Add about ¼ C of water to the bowl and cover with a lid.
• 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:

• 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:

• Cut the sweet potato into cubes.
• 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.
• If the mashed sweet potato is dry, add a little water to help moisten the mixture.
• 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:

• 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.
• 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.
• 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.
• Continue kneading until the dough becomes smooth and elastic.
• 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.
• Once doubled in size, punch down the dough.
• Divide and shape the dough into 12 even balls.
• Place onto a parchment-lined baking sheet (I used a 9" by 13" baking sheet).
• Cover and leave to proof until slightly puffy.
• Towards to end of the proofing time, preheat oven to 375°F/191°C.
• 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.
• Remove from the oven and let cool on a cooling rack.
• Store in an air-tight container for up to 3 days at room temperature. Or freeze for up to 3 months.