To make the dough, place water in a large mixing bowl. Sprinkle yeast on top followed by sugar, then flour, and finally oil.
Fit the mixer with a paddle attachment. Knead to incorporate the ingredients or until there are no dry spots of flour left in the bowl. Stop the mixer and scrape off the mixture from the paddle attachment.
Switch to a dough hook, knead again using medium-low speed until you get a cohesive dough with a smooth outer layer, about 5-8 minutes.
Divide dough into 2 equal portions and cover one with a lid or damp towel to prevent it from drying out.
Knead the other one to remove air bubbles. Roll into an oval, then fold in the sides followed by the top and bottom into a rectangle. Using a rolling pin, press to flatten the dough. Repeat the same step 2 more times. Then, roll it out into a large rectangle (about 18X12), dusting more flour on the work surface as you go. The flour can make the rolling easier and prevent the dough from sticking to the surface. Alternatively, you may roll it out into a rectangle on parchment paper. Note that this may create wrinkles on the dough.
Next, uncover the 2nd dough and make a hole in the middle. Add a spoonful of cocoa powder, and dip your fingers with water. Sprinkle it on the cocoa powder and knead until incorporated. I used 2 tablespoons for extra cocoa flavor.
Now, repeat the same steps and roll into a rectangle as big as the white dough.
Stack the chocolate dough on top of the white.
Lift up the bottom (long side) and roll it up into a swiss roll. Pinch to seal, leaving two ends untouched and cut into 6 portions (for larger buns) or 8 portions (small buns).
Place each bun on parchment paper and transfer to the steamer rack
Fill the steamer pot or pan with water and heat it to about 37C. Place the buns on top and cover with a lid and proof for 30-35 minutes. Check the bun readiness - using your finger, gently poke the bun. If it bounces back, then it's ready. The buns should feel light and soft.
Stick a fork or chopstick in between the lid and steamer to create a gap. Turn on the heat to medium and steam for 18 minutes. Turn off the heat, and let the buns cool down inside the steamer for another 5 minutes before uncovering. Serve warm with a jam or vegan butter.