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 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
Dust the top of the mochi with more cornflour and cut it into 12 pieces of mochi with an oiled knife
Separate the pieces and dust all sides of the mochi with cornflour
Enjoy!