In a large, heavy saucepan melt the sugar over medium-high heat. While many recipes call for swirling the sugar and liquid once the sugar starts to melt I've had a few too many burned moments for that. I also don't want it to crystallize by coming in contact with granulated sugar on the sides so I CAREFULLY pull it towards the middle as it melts, level out the sugar and watch again for liquid and then pull it towards the center again.
Once the majority of the sugar has melted and the color has started to change it's time to watch closely. I switch to a knife at this point so I can break down any sugar pieces that might be a bit large.
As soon as the sugar has all melted and the caramel is about the color of a copper penny, add the butter and whisk while it is melting. It's HOT; use caution.
Turn the heat down to low and slowly pour in the cream while whisking vigorously. The caramel is going to bubble aggressively so be careful (and why a large pan is such a good idea!).
Whisk until all the cream is incorporated and then whisk in the fleur de sel and let it bubble for one minute.
Remove from the burner and set aside to cool. DO NOT PUT THAT SPOON IN YOUR MOUTH. Not just yet at least; you'll burn your tongue!
After 15 minutes you can pour the sauce into jars, over ice cream or onto a spoon for taste testing. :)
You can refrigerate the sauce for up to 2 weeks, but you will need to warm it before use. Warm in the microwave; checking at 10 second intervals or heat in a pot with warm water.