In a heavy saucepan, combine the sugar and water over medium-low heat until the sugar dissolves.
Increase the heat and bring to a boil, swirling the saucepan gently, but not stirring. Simmer until the mixture is syrupy and a deep caramel color, about 5-10 minutes. If you are using a candy thermometer, the sauce should be about 350°F.
Remove the mixture from the heat and whisk in the butter and cream, being mindful that the mixture will spit and sizzle, so be careful. Whisk vigorously until it reaches a smooth, sticky consistency.
Lastly, whisk in the vanilla and fleur de sel. Set aside to cool.
If you’ve tried this recipe, come back and let us know how it was in the comments or star ratings.