Cut the watermelon in half, extract the pulp with a knife or spoon, and blend it with an immersion blender. Don't worry about removing the seeds because you will have to filter everything with a fine mesh strainer, and then they will be eliminated. Weigh the juice obtained until you have about 1 kg.
Add the cornstarch and sugar in a saucepan and then add the watermelon juice a little at a time, stirring constantly to avoid lumps forming. Add some drops of jasmine water, if you like it.
Cook over low heat for about 10 minutes, stirring constantly, until you see it begin to thicken. Continue until boiling and then boil for about 1 minute, then remove from heat.
Let it cool down, then add, if you wish, a few tablespoons of chocolate and mix quickly. I recommend not mixing too much and also the mixture must not be too hot, otherwise the chocolate chips will melt, mixing the chocolate flavor with the watermelon one.
Pour the cream into the single-portion molds and let it cool first at room temperature and then in the fridge for at least 6 hours. You can also serve this fantastic watermelon dessert in glasses, but it is certainly more effective to turn it out onto a serving plate.
Serve the watermelon pudding or Gelo di Melone by decorating it with chocolate chips or chopped pistachios, or both. In all cases, it will be delicious and very nice to see!