Place the starch in a pot, and slowly add the watermelon juice, a little at a time; mix constantly until smooth. Add the sugar; continue mixing. Cook on a medium flame until it comes to a boil, mixing constantly. Lower the flame; then let the mixture thicken a few minutes. Continue cooking, mixing every so often. Add the vanilla, cinnamon and almost all the chocolate (placing aside a few chips for garnish). Mix one last time; then pour the mixture into 6 dessert ramekins. Sprinkle the remaining chocolate chips over the mixture; place in the refrigerator overnight. Just before serving, sprinkle the pistachio granules over each dessert.