Blend the watermelon chunks in batches until smooth.

Strain the puree through a fine-mesh sieve into a bowl, pressing the pulp with a spoon to get all the juice out. Discard the pulp. You should have about 900 ml of juice.

In a 3-quart / 2.8 L saucepan, whisk the cornstarch, sugar, and salt together, then whisk in about 200 ml of the cold watermelon juice until completely smooth with no lumps.

Add the rest of the watermelon juice and the cinnamon stick. Set the pan over medium heat.

Cook, whisking constantly, until the mixture reaches a gentle simmer around 90 C / 194 F and thickens enough to coat the back of a spoon, about 8 to 10 minutes.

Remove the cinnamon stick and take the pan off the heat.

Pour the warm mixture into 6 serving cups or ramekins, about 180 ml each.

Let the cups cool at room temperature, uncovered, for 20 minutes.

Cover and chill for at least 4 hours, or overnight, until fully set.

Before serving, top each cup with chocolate chips, chopped pistachios, and a light dusting of cinnamon if using.