In a heavy saucepan, combine the sugar and water over medium-low heat until the sugar dissolves. Increase the heat and bring to a very low boil, swirling the saucepan gently, but not stirring. Simmer until the mixture is syrupy and a deep caramel color, about 5-10 minutes.
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. Whisky vigorously until it reaches a smooth, sticky consistency. Finally, whisk in the vanilla and fleur de sel. Set aside to cool.
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Notes
Use more or less depending on your tastes. Go slow- you can always add more.