Because many people are leery of corn syrup, you can use Golden or rice syrup in this recipe in it's place. (For those into agave nectar, I haven't tried it, but suspect it may not have the right sugar density for candy making.) If using one of these darker syrups, you'll need to be a bit vigilant and stir it as it's cooking, since it can cook quickly in certain spots of the pan. Your finished caramels will also be darker, too.
I use salted butter. Traditional wisdom was to only use unsalted butter in baking so you could control the amount of salt. But I like the slightly funky taste of salted butter, and if you can find one that's cultured, the flavor is incomparable. If you only have unsalted butter, just add a few extra flecks of salt to the cream.
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 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.
Note: The rice syrup I use is from the Asian market I shop at, found in the Korean foods aisle. I don't know how it compares to rice syrup sold in natural food's stores, but if anyone knows, you are welcome to leave that information in the comments.