Peel and the purple sweet potatoes and cut into small cubes. Steam in a wok around 15 minutes or bake in oven for around 20 minutes. Then mash it with a sticker or scoop.
Prepare warm water around 35 °C and melt the sugar in. And mix the yeast with the water. Mix well and set aside for around 5 minutes.
Prepare the flour in a large bowl. Pour the water with yeast slowly to the bowl with flour and stir with a chop sticker.
Then knead the flour into smooth and soft dough. At the very beginning, it might be a little bit sticky.
Repeat the above steps, mix flour with mashed purple sweet potatoes to make smooth dough too.
Let the two pieces of dough rest for around 1 hour or until the paste ball doubles in size. This process can be done by mixer.
Roll the two pieces of dough into two large and thin rounds. And layer the white round underneath and the purple layer on top. Roll up to a cylinder.
Cut the cylinder into small sections. Brush some oil on the bottom of each section and put them in the steamer.
Bring some water to a boiling and steam for 15 minutes.
Turn down the fire and wait for around 5 minutes. Transfer out and serve hot.