Line a 9 inch square metal baking pan with aluminum foil, leaving at least 1-inch to overlap the top of the pan on all sides. Lightly coat the surface of the foil with butter or nonstick cooking spray. Attach a candy thermometer to a heavy bottomed sauce pan. Place the candy thermometer at least 1/2 inch off the bottom of the pan.
In saucepan, stir together the cream, sugars, and salt. Place the saucepan over medium heat. Stirring constantly, bring sugar mixture to a boil. Once boiling, with a heatproof pastry brush that has been dipped in warm water, wash down the sides of the saucepan to remove any sugar crystals that may have formed. Boil the mixture over medium high heat (do not stir) until the temperature reaches 245 F, brushing down the sides as needed.
Remove from heat and stir in the vanilla extract. Pour the caramel into your prepared pan and let cool.
Remove cooled candy from pan. With a sharp knife, oiled with a tasteless vegetable oil, cut into squares or rectangles. These caramels can be stored at room temperature, between layers of wax paper, for several days.