Put shiratamako in a bowl and knead it 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 0.58 oz (16.5g) each, by rolling the dough between your palms. Then, make a small indentation in the center of each dough ball with your finger.
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 about 5 minutes.
Drain the rice dumplings in a colander and serve them in bowls, topped with anko, kinako, or kuromitsu to taste.