Line a 9 inch square dish with parchment paper. Allow a little overhang so it will be easy to remove the caramel later for cutting. Spray with cooking spray.
In a deep saucepan (at least 4-1/2 inches) bring the sugar, corn syrup, and water to a boil. Cook until the mixture becomes a nice golden brown color. Try not to stir at all. If you need to mix things up, swirl the pan.
While your sugar is browning combine the heavy cream, butter, and fleur de sel in another small pan. Bring just to a simmer. Turn off the heat and set aside.
When your sugar mixture has browned, remove from the heat and stir in your heavy cream using a wooden spoon. Add the vanilla. Return to the burner and cook an additional ten minutes on a medium/low temperature.
Pour your caramels into your prepared pan and allow to cool slightly. Place in the refrigerator for a few hours or overnight.
When you are ready to cut them simply remove from the pan using the parchment as a handle. Sprinkle additional salt onto the top of your caramel slab and press down slightly. Place on a cutting board and slice into pieces. Use wax paper to wrap each individually.