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