Prepare the Watermelon Juice
Cut the watermelon in half and slice into strips. Remove the flesh from the rind.
Add the watermelon flesh to a blender and blend until completely smooth.
Measure out 300 g of watermelon juice for the recipe. Reserve any extra juice for drinks or popsicles.
Make the Mochi Dough
In a large bowl, combine the glutinous rice flour, cornstarch, sugar, and watermelon juice. Mix until smooth and fully combined.
Strain or sift the mixture once to remove any lumps and create a silky texture.
Cover the bowl tightly with plastic wrap and poke a few small holes in the top with a toothpick.
Microwave on high for 5 minutes until the mixture becomes thick and sticky.
Carefully add the butter to the hot mochi mixture.
Let the dough cool to room temperature. Once cool enough to handle, knead until the butter is fully incorporated and the dough becomes smooth and elastic, about 5 minutes.
Shape and Chill
Line a container or tray with parchment paper and transfer the mochi dough into it.
Flatten the surface evenly using your hands or a spatula.
Refrigerate for at least 3 hours or until firm.
Dust the chilled mochi generously with cooked glutinous rice flour to prevent sticking.
Cut into strips and then into bite-sized pieces before serving.