Add the sugar and water to a large saucepan, and place over medium-high heat. Stir constantly until the sugar caramelizes and becomes a deep amber brown color, about 10 minutes. Remove the melted sugar from the heat, and carefully pour it into the bottom of a flan mold. Set aside to cool.
Preheat the oven to 350 F.
In a blender or food processor, combine the milk, condensed milk, eggs, egg yolks, vanilla and cornstarch together. Cream together until smooth, about 1 minute.
Add the heavy cream to the blender, and pulse a few times to incorporate the cream into the mixture. Pour the flan mixture over the cooled caramel syrup in the flan mold. Tap the sides to remove air bubbles.
Place the flan mold in a large baking dish. Create a bain marie by filling the baking dish with water until it comes halfway up the sides of the flan mold.
Place the baking dish in the preheated oven, and allow the flan to bake until the center is firm but still jiggly when handled, about 45-60 minutes.
Remove the flan from the bain marie and allow it to cool completely. Place the flan in the refrigerator for a minimum of 1 hour, up to overnight.
When the flan is set, remove from the fridge. Run a paring knife along the perimeter of the flan to loosen the sides. Place a large plate on top of the flan mold, and quickly invert the plate, so the flan slides out easily.
Serve the flan on a plate using a large spoon. Garnish with whipped cream or fresh berries.
Pro-tip: Transfer the flan onto the plate in front of your guests. It's a lovely touch for them to watch the caramel topping drip down the sides.