Combine the yeast, sugar, and warm water in a bowl. Stir to mix well and set aside for 20 minutes. The yeast mixture will rise and become foamy.
Steam the sweet potato until soft, then discard any water that seeps out after steaming. Using a spoon, mash the sweet potato into a fine purée.
Add the yeast mixture, sweet potato purée, flour, sugar, and oil to the bowl of a stand mixer fitted with a dough hook.
Knead on speed 1 for 6 minutes, until the dough becomes soft and shiny. If the dough is too sticky, add 1 to 2 tablespoons of flour. If you don’t have a stand mixer, you can knead by hand for 20 minutes or until the dough no longer sticks to your hands. Transfer the dough to a lightly floured surface, cover it with plastic wrap, and let it rest for 5 minutes.
On a lightly floured surface, roll out the dough with a rolling pin to form a 14x10-inch rectangle. Roll the dough up into a log, then roll the log back and forth a few times until the surface looks smooth and it reaches a length of 12 inches (30 cm).
Cut the dough into 8 equal-sized pieces using a sharp knife or pastry cutter. Transfer each piece to a 3x4-inch (7 cm x 10 cm) piece of parchment paper. You may roll each piece of dough into a round ball after cutting.
Place the dough balls into a steamer, ensuring there is enough space between each one so they don’t stick together. Cover the lid and let them rise for 60 minutes, or until the dough balls expand in size. Add water to the bottom of the steamer, and you may add 1 teaspoon of Chinese white vinegar to help make the steamed buns whiter. Cover the lid tightly.
Turn the heat to high and steam for 10-12 minutes, or until the dough expands and becomes soft, puffy, and fluffy. Turn off the heat and serve the mantou warm.