Run the watermelon through the food processor or blender until liquefied. Push this mixture through a strainer. You will end up with approximately 2 ½ cups of juice.
Pour the watermelon juice in a large sauce pan, add the sugar and the vanilla. Stir well and cook over medium high heat until it come to a rolling boil.
Place water and cornstarch in a bowl and stir together. Spoon the cornstarch into the puréed watermelon and blend thoroughly.
Maintain the boil stirring continuously, until the mixture starts to thicken about six to eight minutes.
Strain the mixture once again to insure a smooth finish then place in the refrigerator for five minutes until it starts to chill and thicken.
Pour the strained Gelo into martini glasses, top with chopped, toasted pistachios and grated chocolate.
2 pounds seedless watermelon, cut into pieces
½ teaspoon good quality vanilla extract
3 tablespoons cornstarch mixed with two tablespoons water
¼ cup chocolate shavings
¼ cup pistachio nuts, chopped and toasted