Mix together plain flour, cornstarch, instant yeast and sugar, add in milk gradually and start mixing to form a dough.

Add in vegetable oil and knead into a smooth soft dough.

Divide dough into 2 equal portions, 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 chocolate dough on parchment paper, roll out into thin flat dough (cover the rolled dough to prevent it from drying) and keep aside.

Same way put the rested milk dough on parchment paper, roll out into thin flat dough.

On floured kitchen counter place the rolled chocolate dough,on top of chocolate dough placed rolled milk dough.

Roll out again both the dough together to left and right into thinner dough.

From one end start rolling the dough into thin log shape.

Cut into 8 portions using serrated knife, put each on a piece of parchment paper.

Bring 500 ml of water to steamer and boil, turn 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.

After proofing for 25 minutes, turn on gas to high heat until water is boiling (about 3 minutes), then reduce to medium heat and steam for 15 minutes.

Remove the buns from the steamer and serve.