Start by blending the watermelon.
Strain the watermelon juice into a medium saucepan --- you need 2 cups of liquid for this recipe.
Add the sugar, cinnamon powder, and cornstarch; then mix to dissolve both the sugar and cornstarch.
Place the saucepan over medium heat, cover with a lid and bring to a boil.
Lower the heat into medium and use a whisk to mix the watermelon until it thickens. (It typically takes 5 to 7 minutes.)
Pour the watermelon jelly mixture into the molds.
Use an angled spatula (or teaspoon) to even the top of the jelly.
Set aside to cool, then place the molds into the refrigerator for about 3 hours to chill.
Remove the watermelon jelly from the molds and transfer it to a plate.
Garnish with crushed pistachio and chocolate chips, then serve.