Use some water to dissolve the dry yeast, use some water to dissolve the sugar, then add all these to the flour and mix into a dough，take 3/4 of the dough to continue knead, until the dough becomes soft, elastic and has a smooth surface. Add the cocoa powder to the rest of the dough and mix them until the dough gets an even coffee color. Flatten the two dough separately for 10 times each.
Put the cocoa dough on the top of the white dough, roll them together and cut into pieces of 20g each. Lie them on the side so the roll is visible on the top; put two of them together and using chopsticks, pinch them together.
Proof for 45mins at 30-35°C.
Put the mantou in the steaming basket, and steam for 9mins over high heat.