Wash the purple sweet potatoes thoroughly. Place them in a pressure electric cooker and add water until it reaches half the height of the potatoes. Seal the lid and set to cook for approximately 10 minutes.
Remove the cooked purple sweet potatoes and peel them carefully (caution: hot). Mash the potatoes into a smooth paste using a spoon in a container. If unavailable, substitute with pumpkin (note: pumpkin releases more water) and mash similarly into a paste.
Proceed to knead the dough. The ratio of purple sweet potato to flour (glutinous rice flour to sweet potato starch is 1:2) should be about 2:1. Due to the dryness of purple sweet potatoes, add milk gradually in small amounts during kneading. For pumpkin, omit milk as it naturally contains more moisture.
Wear gloves and knead the dough repeatedly to achieve a chewy texture. Roll the dough into long strips, cut into small pieces, and shape into balls. Cook in a large pot of boiling water for about 3-4 minutes until the balls float to the surface.
The crucial step is to rinse the cooked balls in cold water multiple times (recommended: 4 times) to enhance chewiness. Serve with preferred ingredients. For storage, freeze excess balls individually in the freezer before placing them in a sealed bag.