In a small bowl, slowly mix in a little pureed watermelon into the cornstarch until dissolved.
In a small saucepan, mix together watermelon puree, dissolved cornstarch, sugar, vanilla extract, and cinnamon stick. Bring the mixture to a boil and reduce heat to low. Simmer, stirring constantly, until thickened, 10-15 minutes.
Rinse 2 large or 4 small ramekins with water. Do not dry. Pour the thickened watermelon into the wet ramekins and refrigerate overnight.
Serve chilled in the ramekins or unmold them. Sprinkle with ground cinnamon, chocolate chips, and pistachios.