Steam the sweet potato slices over boiling water 18 to 20 minutes, until a fork goes through with no resistance. Do not boil them.

Mash the potato while still hot, as smooth as possible. You want about 300 g of cooked mash.

Stir the sugar and salt into the hot mash until dissolved. Add the glutinous rice flour and about three quarters of the tapioca starch and work it together until cool enough to handle.

Knead 3 to 4 minutes, adding the remaining tapioca a spoonful at a time, until the dough is soft, holds a clean fingerprint and does not stick to your palm. Add hot water 1 teaspoon at a time only if it cracks.

Roll into ropes ¾ inch thick, cut into ¾ inch pieces, and roll each into a ball. Dust lightly with tapioca starch and rest uncovered 10 minutes.

Heat the oil to 285F. Slide the balls in and leave them undisturbed 2 minutes, until they rise and float.

Once every ball is floating, press them with a wide flat spider or slotted spoon, flattening each ball to about half its height. Press every 15 to 20 seconds, working across the whole surface, for a total of 6 to 8 minutes.

The balls get visibly larger, lighter in the spider, and spring back faster after each press. That instant spring back means they are hollow.

Raise the oil to 340F for a final 45 seconds to firm the shell, then drain on a wire rack. Toss with plum powder, seaweed salt or five spice sugar if using, and eat within 20 minutes.