Peel and slice the purple yam. Steam and mash the purple yam.
Mix the instant yeast into water/soymilk, along with the sugar. Cover the container with a lid and rest for 10 minutes.
Pour the yeast mixture to the mashed purple yam. Mix well.
Place the all-purpose flour in a big bowl. Pour the purple yam mixture into the flour. Mix well using chopsticks.
When the mixture thickens, sprinkle some flour on the countertop and start to knead it into a dough.
Add the cooking oil to the dough. Continue to knead until the dough fully absorbs the oil.
When the dough becomes elastic, it is ready for rising. Cover the dough with a lid and let it rest for 10 to 15 minutes.
Sprinkle some flour into the dough to prevent stickiness. Divide them into 20g per piece. Bring out 5 balls at once, and cover the unused balls with a lid to prevent dryness.
Roll the divided dough into balls and flatten them into a disc.
Each mantou needs 5 discs. The first disc should have the longest diameter. Place the second disc over half of the first disc, and we will do the same for the third and fourth disc. Please ensure the diameter of the discs is the same.
Roll up the fifth disc into a tube shape. Please ensure again that its length is the same as the diameter of other discs.
Slowly roll the other discs over the tube.
Slide the finished product in half. Gently fold the outer dough as petals. Place the mantou directly in the steamer and cover with the lid.
The mantous need to rest for another 30 minutes. Do not touch them until they are cooked.
Steam the mantous using medium-high heat for 10-12 minutes. Do not open the lid when steaming.