2. Peel and cut the purple potato, steam it, and sieve it into purple potato paste.
3. The materials in the purple potato dough are evenly mixed, and a small amount of water is added for many times to knead into a smooth dough.
4. The dough is rounded and fermented in a warm place to double its size. After the fermented purple potato dough is taken out, add flour to the chopping board and knead it continuously until there are no air holes after the dough is cut.
5. Mix flour and baking powder in the white dough and knead it into a smooth dough. Knead the dough into a circle, put it in a pot and ferment it in a warm place until it is twice as big. After the fermented white dough is taken out, flour is continuously added to the chopping board and kneaded until there are no air holes after the dough is cut.
6. Roll two doughs into cakes with the same thickness with a rolling pin, put them together and brush a little water in the middle.
7. Roll up the pasted dough from top to bottom and roll it into a cylinder.
8. Cut it evenly with a knife and put it in a steamer to wake it up 15 minutes. After the fire boils, steam it on medium heat for 15 minutes, then turn off the fire, stew for another 5 minutes and then uncover the lid.