Combine the yeast, sugar and warm water in a bowl. Stir to mix well. Set aside for 20 minutes. The yeast mixture will rise and becomes foamy.

Steam the sweet potato until soft, discard the water that seeps out after steaming. Using a spoon, mash the sweet potato into a fine puree.

Add the yeast mixture, sweet potato puree, flour, sugar and oil in the bowl of a stand mixer attached 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 - 2 tablespoons of flour. If you don't have a stand mixer, you can knead with hands for 20 minutes or until the dough doesn't stick to your hands.

Transfer the dough out and cover it with plastic wrap and let rest for 5 minutes.

On a slightly floured surface, roll out the dough with a rolling pin to form a 14x10" rectangle.

Place the dough into a steamer. Cover the lid of the steamer and let rise for 60 minutes, or until the dough balls expand in size. Make sure you leave enough space between each dough ball so they don't stick together.

Add water to the bottom of the steamer. You may add 1 teaspoon of Chinese white vinegar to make the steamed buns whiter. Cover the lid tightly.

Turn on high heat and steam for 10-12 minutes, or until the dough expands to soft, puffy and fluffy mantou. Turn off the heat and serve the mantou warm.