Add the sugar in an even layer over the bottom of a heavy, 2-3 quart saucepan.
Heat the sugar over medium-high heat, whisking it as it begins to melt. The sugar will form clumps, just keep whisking and they will melt. Stop whisking once all of the sugar has melted, swirl the pan occasionally while the sugar cooks.
Continue cooking until the sugar has reached a deep amber color. At this point caramel can burn in a matter of seconds, so keep a close eye. If you have an instant read thermometer, cook until the sugar reaches 350 degrees F.
As soon as the sugar reaches 350 degrees F, add the butter all at once. Be careful as the caramel will bubble when the butter is added. Whisk the butter into the caramel until it is completely melted.
Remove the pan from the heat and slowly pour in the cream. Again, be careful as the mixture will bubble.
Whisk until all of the cream is incorporated and the sauce is smooth. Add the fleur de del and whisk to incorporate.
Set the sauce aside for 10 to 15 minutes to cool, then pour into a glass container and cool to room temperature. Sauce can be store in refrigerator for up to 2 weeks. Warm sauce before using.