Heat the oil in a large non-stick skillet over medium heat. Once hot, add the minced shallot, lemongrass, garlic, green onions, and chili if using. Sauté for about 2 minutes.
Drain and finely cut the shiitakes and black mushrooms and add to the skillet. Sauté for another 2 minutes.
Add the shredded sweet potato and 2 tablespoons of water. Cook for 7-10 minutes, or until the sweet potato is tender.
In a small bowl, combine the soy sauce, water, cornstarch, sugar, and black pepper. Add to the skillet and stir to coat. Cook for 1 more minute. Remove from heat and let cool for about 15 minutes. Scoop out about 3 tablespoon of the sweet potato filling and squeeze it into a ball using your hands (you can also use a cookie scoop to make it easier). Transfer to a plate and repeat with the remaining filling.
Punch the dough down and divide it into 8 balls (6 if you want larger buns). Cover with plastic film to prevent them from drying. Take one ball and flatten it into a circle of about 4-inch diameter. Place the sweet potato filling in the middle of the dough and wrap the dough around it. Check out this video to get an idea on how to wrap it. Repeat with the remaining dough and filling.
Bring a pot of water to a boil and place your bamboo steamer over it. Depending on the size of your steamer, place 3-4 buns inside, leaving about 2-inch between each (they will rise). Steam for 17-20 minutes. Remove from heat and repeat with the remaining buns.
Enjoy warm, or at room temperature! These buns will keep for up to 3 days in the refrigerator. Reheat in the steamer for 7-10 minutes.