Peel the purple yam with a peeler. Cut into pieces, so it can be cooked through easily.
Put the chopped purple yam into a bowl and to the steamer. Steam for 10-15 minutes or when a fork can poke through easily.
Mashed the purple yam using a fork. Add sugar. Mix well. Adding sugar is optional, but I want it to have a hint of sweetness in the pearls.
Take out 100g mashed purple yam and add 1/3 of the tapioca starch at a time. Knead until all starch disappears. If the dough is too dry, add water by teaspoon before kneading. The dough should not be sticky, and the purple yam has its own moisture, so make sure to add water only when necessary.
Prepare a bowl filled with a small amount of tapioca starch (2 tsp). Set aside.
Use your fingers to roll each of the small dough into a ball. Put the pearl into the bowl so it won’t stick with other pearls. Repeat this step until the dough is used up.
Boil a pot of water. Add the pearls and let it boil for 1-2 minutes. When they are floating, they are ready.
Put the cooked pearls in a ice water bath, so it can be more bouncy and chewy.
Serve the pearls with milk and ice.