For the filling:
Wash the red beans and soak overnight (this will shorten the cooking time).
The next day, boil the beans for 1-1.5 hour letting it simmer on a low heat until the beans are soft. Drain and blend into a smooth paste.
Stir in the sugar and the oil. Reheat in a small pot until the flavour comes out a little bit. Set aside. You can store this in the fridge for up to a week.
For the bun:
Mix the yeast with 2tbsp of warm water, 1tsp sugar, and 1tsp of bao flour in a small bowl. Leave it in a warm place for about 10 minutes until it bubbles up (i.e. yeast is active). My kitchen was quite cold so I had to leave the bowl in a larger bowl of warm water to ensure the yeast grows.
Sift the bao flour and self-raising flour into a large bowl. Stir in the sugar, the yeast mixture, warm water, and the melted butter. Mix until it becomes a soft dough with a spoon.
Flour a work surface with bao flour and knead until smooth and elastic, approximately for 10 minutes. The dough will be quite soft and sticky so be prepared to add quite a bit of bao flour when kneading so it doesn’t stick to your hands. I felt like I was fighting against the dough to not stick to my hands.
Prepare an oiled bowl and place the dough into it, cover, and let it rise for an hour or so or until the dough as doubled. Heat will play a large part in how quickly the dough rises and since it was cold in my kitchen, I had to turn on the heater near the bowl to make it warm enough to rise in an hour.
Remove the dough from the bowl and knead for about five minutes on a floured surface. Divide it up into 12 balls.
Heat up a wok with a bamboo steamer until the water boils rapidly. Meanwhile, flatten each ball in the palm of your hands and fill up with a teaspoon of the red bean paste. Close up the ball and place it with the sealed side to the bottom and place on top of a baking paper square. Ensure adequate space between the buns when laying out on the steamer as they will expand during steaming.
Steam the buns for 10-15 minutes on high heat. Try not to over-steam as the buns will start to get little brown spots (steam burns).
Serve hot. You can also refrigerate/freeze and re-steam later.