Mix together plain flour, cornstarch, instant yeast, and fine sugar, add in milk gradually and mix into a dough.
Add in vegetable oil, mix until well blended.
Divide dough into 2 equal portions, and cover 1 portion (for chocolate dough) to prevent it from drying out.
Knead the 1st portion (milk dough) for 6-7 minutes until the outer is smooth (lightly coat hand with flour if it is sticky), then cover and set aside to rest for 6-7 minutes.
Add cocoa powder to 2nd portion gradually, knead for 6-7 minutes until the outer is smooth, then cover and set aside to rest for 6-7 minutes.
Put the rested milk dough on parchment paper, roll out into thin flat dough (cover the rolled dough to prevent it from drying).
Put the rested chocolate dough on parchment paper, roll out into thin flat dough.
Put chocolate dough on top of milk dough, roll out again to left and right into thinner dough.
Remove the parchment papers, put the thin dough on floured working table, flatten the dough at the end where rolling starts, then roll up the thin dough into a rod shape.
Cut into 8 portions using serrated knife (for a nicer cut), put each on a piece of paper.
Bring 500 ml of water to the boil in a steamer, off the heat, then add in 1 Litre of plain water.
Arrange mantou dough in steamer racks (leave some space between each), then put the racks over the warm water, cover and proof for 25 minutes. (Reminder: Using a bamboo steamer to steam will prevent condensation water from dripping onto mantou, thus yield a prettier bun.)
After proofing for 25 minutes, turn to high heat until water is boiling (about 3 minutes), then reduce to medium heat and steam for 15 minutes.
Chocolate spiral mantou is done eat fresh or store in the refrigerator.