Combine the joshinko, shiratamako, and sugar in a bowl and knead while gradually adding water. If the dough doesn't come together, add a small amount of water (about ½ Tbsp) and continue adjusting until the dough comes together properly.
Once the dough reaches a soft, earlobe-like consistency through kneading, shape it into dumplings that are around 0.46 oz. (13g) each, rolling the dough between your palms.
Fill a pot with plenty of water and bring it to a boil. When it comes to a boil, add the dumpling dough to the pot and boil it over medium heat for about 4 minutes until it rises to the surface. After all the dumpling dough floats to the surface, continue boiling for a few more minutes.
Fill a bowl with ice water or cold water. Transfer the boiled dumplings to the bowl and let them cool for a few minutes.
Drain the dumplings in a sieve. Wait a few minutes for their surface to dry, and then skewer 4 dumplings per bamboo skewer.
Using a frying pan, kitchen torch, or direct flame, lightly brown the dumplings on both sides.
Mix the starch with water until it dissolves. In a pot, combine sugar, soy sauce, and water, and bring it to a gentle boil over low heat. Gradually add the mixture of starch and water while stirring to thicken the glaze.
Serve the skewered dumplings on plates and drizzle the sweet soy glaze over them.