1. Line a 9-inch (23 cm) loaf pan with foil and spray the inside with cooking spray.
2. Heat the cream with 2 tablespoons of the butter in a small saucepan with the vanilla and 1/2 teaspoon sea salt until the mixture begins to boil. Remove from heat, cover, and keep warm while you cook the syrup.
3. In a medium, heavy duty saucepan (4 quarts, 4l), fitted with a candy thermometer, heat the corn syrup, golden, or rice syrup with the sugar, and cook, stirring gently, to make sure the sugar melts smoothly. Once the mixture is melted together and the sugar is evenly moistened, only stir is as necessary to keep it from getting any hot spots.
4. Cook until the syrup reaches 310ºF (155ºC).
To get an accurate reading while the syrup is cooking, tilt the saucepan to make sure the bulb of the thermometer is fully submerged in the syrup, tilting the pan if necessary.
4. Turn off the heat and stir in the warm cream mixture, until smooth.
5. Turn the heat back on and cook the mixture to 260F (127C).
6. Remove the pan from the heat, lift out the thermometer, and stir in the cubes of butter, until it’s melted and the mixture smooth.
7. Pour the mixture into the prepared loaf pan and wait ten minutes, then sprinkle 1/4 teaspoon of the sea salt over the top. Set on a cool rack and let cool completely. Once cool, lift out the foil with the caramel, peel away the foil, and slice the bar of caramel with a long, sharp knife into squares or rectangles.