(When using fresh edamame) Bring a pot of water to a boil. Add edamame to the pot and cook for about 5 minutes over medium-low heat until tender. Then, drain in a colander and let cool.(When using frozen edamame) Thaw edamame. Remove the beans from the pods and peel off the thin skins covering each bean.
Mash the edamame beans to a slightly coarse consistency using a mortar and pestle or a blender (you can also mash them completely if you prefer). Mix water, sugar, and salt into the mashed edamame. Zunda paste is ready.
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 0.7 oz (20g) 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 5 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 the zunda paste.