Fill the steamer with tap water.
Make the white mantou dough first to avoid colour contamination. Mix the dry ingredients for the white dough well, then add the water. Using a dough hook attachment, knead in a stand mixer on low for approximately 8 minutes. Different brands of flours have different hydration levels so you may need more water to get a smooth dough: start with 125g of water then gradually add more, 1 teaspoon at a time, if necessary.
You will know that the dough has been sufficiently kneaded once it becomes a smooth ball that does not stick to the mixing bowl or to your hands. After stopping the stand mixer, knead with your hands for another minute or so. (Alternatively, if you've missed your gym workout, you can do the kneading by hand- just remember to rest the dough for 10-15 minutes after the ingredients have combined to form a rough mixture.)
Roll the dough into a ball, put it in a bowl (covered) then refrigerate to stop the proofing process whilst you work on the green pandan mantou dough.
Repeat Steps 2-3 using the green pandan dough ingredients (where pandan juice is substituted for water).
Roll the green pandan dough on a well-floured surface, parchment paper or Silpat, using a well-floured rolling pin, to make sure the dough doesn't stick. You want to achieve a thin rectangle shape. (The thinner the dough, the more swirls there will be in your pandan spiral mantou buns.) If you're still having problems with the dough, sandwich the dough between 2 layers of parchment paper (1 above the dough and 1 below) before rolling. (If the dough still sticks, your dough is under-kneaded or over hydrated.)
Once you've rolled out the green dough, take out the white dough from the fridge and roll it out, also on a well-floured surface/ parchment paper/ Silpat. Make sure it's rolled to a rectangle of the same size as the green dough then place the green dough on top of the white dough. (This will give swirled pandan mantou buns with a white exterior. For buns with a green exterior, place the green dough underneath the white dough.)
With the green dough on top of the white dough other, lightly roll them with the rolling pin to get thinner spiral layers.
Roll the doughs into a log, like a Swiss roll. When rolling the doughs into a log, start from the uglier edge (the edge which is less straight) and make everything is held together tightly.
Use a serrated knife and cut the long log into 16 pieces, placing each 1 on a small piece of parchment paper/ cupcake wrapper.
Place the mantous into the steamer, ensuring that there is at least 1.5 inches in between each mantou if you don't want to stick to each other after steaming. Allow to rest for 20 (tropical weather of 30C/ 86F) to 30 minutes (25C/ 77F weather). You will know the dough is sufficiently rested when the buns are very smooth, have increased about 40-50% in size (they will not double in size) and spring back after being pressed. If you're unable to steam all the mantou buns at 1 go, store them in the fridge after the 20-30 minutes to stop them from overproofing- this is particularly important in warm countries as heat expedites the proofing process.
Turn on the fire to high, then reduce to medium-low once the water is at a rolling boil. After 10 minutes, switch off the fire. The mantous are best enjoyed warm. If the mantous aren't as smooth as you'd like, scroll back up my mantou troubleshooting section which identifies common mantou making mistakes. (More details are provided in my matcha mantou post.)