Bring the water and ginger (if using) to boil in a small pot. Add in the sugar, stir until sugar has dissolved and remove from heat. Let cool completely. The syrup can me made a day in advance and stored in the refrigerator.
Steam and Mash the Sweet Potato
Peel the sweet potato and cut them into chunks. Steam the sweet potato for 15 to 20 minutes until they are soft.
Transfer the sweet potatoes into a sieve to drain away excess moisture.
Use a scraper and mash the sweet potato through the sieve. This process is a little tedious but it helps to get a fine and smooth puree without lumps.
Making the Dough and Balls
Once all the sweet potato is mashed, measure out300 grams of sweet potato puree into a large mixing bowl. The remaining sweet potato puree can be used as a sandwich filling, pancake topping or eaten just like that!
Whisk the tapioca flour and glutinous rice flour together in a small bowl. Add 80% (just roughly estimate) of the flour mixture into the sweet potato puree and mix very well with a spatula, adding more flour mixture if needed to get a soft dough that does not stick to the spatula or your hands. Test if the dough is ready by taking a small piece and roll it out into a log between your hands, if it doesn't stick, it's ready. If you need more flour, mix in additional tapioca flour and glutinous rice flour in the ratio 1:1 (e.g. 10 grams tapioca flour + 10 grams glutinous rice flour). Don't add to much flour otherwise the dough will crack when you roll it out.
Once the dough is ready, dust the table top with tapioca flour. Take a small portion of dough and roll it out into a log about 2-cm in diameter. Cut them into 2-cm pieces, dust the cut pieces generously with tapioca flour (so they won't stick to one another) and set them aside.
Repeat until all the dough is used up. The beauty of sweet potato balls is that you don't have to shape the balls, they can be left a little irregular in shapes and sizes.
Freezing Uncooked Balls
Freeze whatever you are not planning on eating on the day in an airtight container lined with parchment paper and dusted with tapioca flour. Cook the frozen sweet potato balls the same way as below.
Cooking the Balls
Bring a large pot of water to boil over high heat - don't use a small pot as it will longer for the water to come back to a boil. While waiting for the water to boil, get ready a large bowl of ice cold water.
Add in the sweet potato balls and stir immediately to make sure they don't stick to each other and to the bottom of the pot.
Cook over high heat, stirring frequently, until the balls float to the top. Let it boil for another 2 minutes. The balls will take about 5 minutes in total to cook (about 7 if they are frozen).
Strain the balls and immediately transfer them into the ice water - add more ice if needed to keep the water cold. Let the balls sit in the water for 10 minutes. Strain the balls and add into the cooled/cold Ginger Syrup and steep for at least 30 minutes before serving.
The sweet potato balls are best consumed on the day they are cooked, but can be stored in the refrigerator (in the syrup) for up to 2 days (the texture will be a little different though).
The sweet potato balls are best with green bean soup, soy pudding, grass jelly etc.
The sweet potato balls will have a glutinous rice flour aftertaste immediately after they are cooked, but once steeped in the syrup, the floury taste will be gone.
The sweet potatoes can be swapped with taro (yam).