1. Add water and purple potato mash to the egg yolk and stir well
2. Add vegetable oil and mix well
3. Add the sifted low-gluten flour and stir evenly with a whisk in irregular directions
4. Add soft white sugar and stir evenly in irregular directions
5. The egg whites are added three times with soft sugar and beat until dry and foamy
6. First take 1/3 of the egg whites and add them to the egg yolk paste, stir and mix well, then add the remaining egg whites and stir evenly
7. Add purple sweet potato diced and mix well
8. Shake out the bubbles, put it in a preheated oven at 180 degrees, and fire up and down at 180 degrees for about 50 minutes.
9. Bake until the surface is colored and covered with tin foil, and it will be inverted immediately after it is out of the oven. After it is completely cooled, it will be demoulded
