In a medium bowl, combine the glutinous rice flour and granulated sugar. Use a whisk to mix them together thoroughly until well combined.
Gradually pour in the water and fresh watermelon juice, continuously stirring the mixture until it becomes a cohesive batter free of lumps.
Transfer the batter into a microwave-safe bowl or dish. Cover the bowl with a damp kitchen towel to retain moisture during cooking.
Microwave the mixture on high for 2 to 3 minutes. Keep an eye on it, stopping halfway through to stir, which promotes even cooking and helps the mochi become translucent and slightly puffy.
Once cooked, carefully remove the hot dish from the microwave and allow it to cool for a few minutes until it's safe to handle.
On a clean surface, generously dust with cornstarch to prevent sticking, then gently transfer the mochi dough onto the surface.
Using a knife or dough cutter, divide the dough into small portions of approximately 1 tablespoon each. Flatten each portion into a disc shape.
In the center of each disc, place a small cube of fresh watermelon and, if desired, a dollop of sweetened red bean paste. Fold the edges of the disc over the filling, pinching them together to seal securely.
Lightly roll each filled mochi ball between your palms for a smooth appearance.
Finally, dust the completed mochi with additional cornstarch to prevent any sticky surfaces.
Notes
Serve chilled for the best flavor experience.
Keyword dessert, Japanese sweets, mochi, watermelon