Start by cutting a ripe watermelon into chunks and blending until smooth. Strain through a sieve to obtain 1 cup of watermelon juice.

In a mixing bowl, whisk 1 cup of glutinous rice flour and granulated sugar. Gradually add the fresh watermelon juice, stirring until smooth.

Transfer the mixture to a microwave-safe bowl and microwave on high for 1 minute. Stir every minute until the dough is glossy and translucent, about 4-5 minutes total.

Dust a plate with cornstarch. Transfer the hot mochi mixture onto the plate, let it cool for 10 minutes, then shape into balls or discs.

Dust shaped mochi with cornstarch to prevent sticking. Serve immediately or store in an airtight container in the fridge for up to 2 days.