[Below is a re-posting of David's recipe originally posted here in 2008.]
The first step is to make invert sugar to prevent the sugar in the caramel from spontaneously crystallizing.
Salted Caramel Invert Sugar
3 C Sugar
1.5 C Water
1/4 t Citic acid OR juice of 1/2 lemon
Put ingredients in a non reactive pot and bring to a boil. Lower heat and simmer for 30 minutes.
2 C Sugar
1 oz Invert Sugar
1.25 C Cream, warm
1 oz Butter
Fleur de sel
Put invert sugar and sugar in a wide high sided non reactive pot on high heat. Every minute or so slowly mix in granulated sugar with some that is liquefied. Eventually you will have a paste. Warm Cream separately.
Continue to cook sugar until it begins to caramelize. Using a candy thermometer monitor the temperature of the cooking sugar. The classic caramel stage is around 330-350 degrees F. You can cook it longer for a less sweet more bitter sauce. Do not go above 390 F.
When your desired temperature is reached turn off the heat and slowly and very carefully add the warmed cream in small increments. When the cream is fully incorporated, turn the heat on high and heat the caramel to 230 F. This will go quite quickly. Turn off heat and add the butter. Stir until the butter has completely melted. Add your desired amount of Fleur de sel or other sea salt. Let cool.
It will store in the refrigerator for up to 4 months.