Begin by cutting a ripe watermelon into chunks, discarding any seeds. Blend the chunks until smooth in a blender, then strain the mixture to extract about 1 cup of fresh watermelon juice.

In a medium mixing bowl, whisk together 1 cup of glutinous rice flour and about ¼ cup of granulated sugar until well combined. Gradually pour in the watermelon juice creating a smooth, lump-free batter.

Transfer the watermelon mixture to a microwave-safe bowl and cover loosely with a damp paper towel. Microwave in 1-minute intervals, stirring thoroughly, for a total of 4 to 5 minutes until glossy and translucent.

Dust a large plate with cornstarch and transfer the hot mochi mixture onto the plate. Let it cool for about 5 to 10 minutes until it's warm but manageable. Shape the mochi into small balls or discs.

Lightly dust each piece of Watermelon Mochi with cornstarch to prevent sticking together. Serve immediately or store in an airtight container.