To make the watermelon juice place watermelon flesh in a blender and blitz until smooth, then pass through a fine mesh sieve to remove any pulp.
Combine the glutinous rice flour, sugar, and cornstarch in a bowl.
Add the watermelon juice and oil, and whisk until smooth.
Heat a deep frypan or saucepan over medium heat and add the mochi mixture.
Heat, stirring consistently, for 5-8 minutes, or until the mochi mixture thickens and changes into a dark pink translucent color (the mixture should be very sticky).
Remove from the heat.
Prepare a 12cm x 12cm square mold and line it with parchment paper, if you don’t have that size any small rectangular container should work.
Pour the warm mochi into the lined mold and press it down the spatula to flatten it.
Place the mochi in the fridge to cool for 1-2 hours, or until slightly firmed up.
Dust your work surface with cornstarch.
Remove the mochi from the mold and place it on the dusted surface.