Add the sugar in an even layer over the bottom of a medium (3 to 4-quart) saucepan. Heat the sugar over medium heat, whisking it as it begins to melt. You'll see that the sugar will begin to form clumps, but that's okay. Just keep whisking and as it continues to cook, they will melt back down. Stop whisking once all of the sugar has melted, and swirl the pan occasionally while the sugar cooks.
Continue cooking until the sugar has reached a deep amber color. It should look almost a reddish-brown, and have a slight toasted aroma. This is the point where caramel can go from perfect to burnt in a matter of seconds, so keep a close eye.
As soon as the caramel reaches its sweet spot, add the butter all at once. Be careful, as the caramel will bubble up when the butter is added. Whisk the butter into the caramel until it is completely melted. Remove the pan from heat.
Slowly pour the cream into the caramel. Again, be careful because the mixture will once again bubble up ferociously.
Whisk until all of the cream has been incorporated and you have a smooth sauce. Add the fleur de sel and whisk to incorporate.
Set the sauce aside to cool for 10 to 15 minutes and then pour into your favorite glass jar and let cool to room temperature. The sauce can be refrigerated for up to 1 month.
Salt - I highly recommend using flaky sea salt like fleur de sel or Maldon sea salt. If you need to substitute table salt, use only 1 to 1½ teaspoons, or the sauce will be much too salty!
Saucepan - It is important to use a saucepan of at least 4-quart capacity. It will seem too big for the job, but when the butter and cream are added, the sauce bubbles up ferociously, and you need that space so it doesn't boil over.
Storage - The caramel sauce can be kept in the refrigerator for up to 1 month. This recipe is not suitable for canning or long-term storage.
Freezing - The sauce can be stored in an airtight container in the freezer for up to 3 months. Thaw overnight in the refrigerator before using it.
Reheating Instructions - To reheat the sauce, remove the lid from the jar and microwave for about 45 seconds, then stir. It should be smooth and pourable, but not piping hot.