Peel the purple yam and cut it into pieces for easier cooking.
Place the chopped yam in a bowl and steam for 10-15 minutes, or until a fork can easily pierce through.
Mash the steamed yam using a fork and mix in sugar (optional, for a hint of sweetness).
Measure out 100g of the mashed yam and gradually add 1/3 of the tapioca starch at a time, kneading until fully incorporated. If the dough is too dry, add water by the teaspoon. The dough should be non-sticky, as the yam's moisture should suffice—add water only if necessary.
Prepare a bowl with 2 tsp of tapioca starch and set it aside.
Roll small portions of the dough into balls using your fingers. Place each ball in the prepared bowl to prevent sticking. Repeat until all the dough is used.
Boil a pot of water and add the pearls, boiling for 1-2 minutes until they float.
Transfer the cooked pearls to an ice water bath for a bouncy and chewy texture.
Serve the pearls with milk and ice.