To make the caramel that will eventually be the topping on the flan, put 1/2 cup of the sugar in an 8-inch cake pan and heat it on top of the stove over medium-low heat. Holding the edge of the pan with a hot pad, tilt the pan from side to side as the sugar melts and caramelizes. When all the sugar has melted into a dark brown liquid, remove the pan from the stove. Tip the pan so that the sides as well as the bottom are coated with the syrup. Set the pan aside. Peel and seed the persimmons, then cut them into chunks. Place them in a small saucepan and cook them over medium heat for 5 minutes, stirring often. Puree the fruit in a blender, then strain it through a wire-mesh sieve. You should have about 3/4 cup thick puree. Discard any fibers in the strainer and set the puree aside. Preheat an oven to 325 F. Heat the milk and cream together in a saucepan over medium heat until bubbles form around the edges. At the same time, put on a kettle of water and bring it to a boil. In a bowl beat the eggs lightly and then add the remaining 1/2 cup sugar, the salt, and vanilla,; When the milk-cream mixture is ready, slowly pour it into the egg mixture, stirring continuously. Stir in the pureed fruit. Put the caramelized cake pan in a shallow roasting pan large enough to hold it. Pour the custard into the cake pan; it should fill it to the rim. Pour the boiling water into-the roasting pan until it reaches halfway up the sides of the cake pan. Bake the custard in the preheated oven until a knife inserted into the middle of the flan comes out clean, 35 to 45 minutes. Remove the flan from the oven and let it cool to room temperature. At this point it can be served, or it can be refrigerated for several hours. To unmold the flan, slide a thin-bladed knife or spatula around the edge of the pan to loosen it. Invert a shallow serving plate on top of the flan and, holding the flan and the serving plate firmly together, flip them. The flan pan will now be on top and should lift off. If the flan didnt unmold on the flip, give the pan a shake to encourage it. Makes 8 to 10 servings