Combine joshinko, shiratamako, and sugar in a bowl and knead the mixture 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 forms properly.
Once the dough reaches a soft, earlobe-like consistency through kneading, shape it into small balls, about 0.46 oz (13g) each, by rolling the dough between your palms.
Fill a pot with plenty of water and bring it to a boil. Once boiling, add the dough balls and cook them over medium heat for about 4 minutes, until they rise to the surface. After all the dough balls have floated, continue cooking for a few more minutes.
Fill a bowl with ice water or cold water. Transfer the cooked dough balls to the bowl and let them cool for a few minutes.
Drain the rice dumplings in a colander. Wait a few minutes for the surfaces to dry, and then skewer 4 dumplings on each bamboo skewer.
Using a frying pan, kitchen torch, or direct flame, lightly brown the dumplings on both sides.
Mix starch with water until it dissolves. In a small pot, combine sugar, soy sauce, and water, and bring it to a gentle boil over low heat. Gradually add the starch mixture while stirring to thicken the glaze.
Serve the skewered rice dumplings on plates and drizzle the sweet soy glaze over them.