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 35g each.
Proof for 50mins at 30-35°C.
Put the mantou in the steaming basket, and steam for 10mins over high heat.