Summarize:

When the sauce has finished boiling, remove the pan from the stove. Stir in 1 teaspoon (6 g) of sea salt until it's well combined and the sauce is smooth once again. Fleur de sel has a very delicate texture so it easily melts into the sauce, but any fine sea salt will work for the recipe. Make sure to mix in the salt when the sauce is still hot so it completely dissolves into the mixture. Once you've mixed the salt into the caramel sauce, leave it in the pan and set aside. Let the sauce cool for approximately 15 minutes before transferring it from the pan. If you plan to use the sauce right away, serve it while it's still warm so it isn't too thick to pour. When the sauce has cooled sufficiently, carefully pour it into a glass jar for storage. You can store the sauce in the refrigerator for up to two weeks. If you are refrigerating your sauce, be sure to warm it up before serving or incorporating it into another dish. You can heat it in the microwave on high in 5 second intervals until it's thin enough to pour.
Remove the pan from the heat and mix in the salt. Allow the sauce to cool. Pour the sauce into a glass jar for storage.