Wash and peel 100g of purple sweet potato.

Cut the sweet potato into thick slices.

Place the slices in a steamer and steam for 15 minutes until soft.

Transfer the steamed sweet potato to a bowl, discarding any water that may have come out during steaming.

Add 10g of white sugar to the sweet potato.

Mash the sweet potato and sugar mixture into a smooth paste.

Shape the paste into small balls, each weighing about 10g. Set aside.

In a bowl, add 120g of tapioca starch.

Slowly add 70g of boiling water to the starch, a little at a time, while stirring continuously with chopsticks.

Continue adding the water in small amounts and stirring until the mixture forms a cohesive dough.

Put on gloves and knead the dough in the bowl until it starts to come together.

Transfer the dough to a work surface and knead until it becomes smooth and elastic.

Roll the dough into a long strip.

Cut the strip into 15g pieces.

Flatten each piece into a thin sheet using your hands or a rolling pin.

Place a ball of purple sweet potato filling onto the center of each dough sheet.

Fold the dough over the filling and pinch the edges to seal it.

Roll the filled tang yuan gently to smooth out the surface.You can dust them with some dry tapioca starch to prevent sticking.

Bring a pot of water to a boil.

Add the tang yuan to the boiling water one by one.

Cook until all the tang yuan float to the surface.

Continue cooking for an additional minute.

Remove the tang yuan with a slotted spoon and transfer to a bowl.

Serve the crystal tang yuan warm or at room temperature, optionally with a sweet syrup or ginger soup.