Melt the sugar in a large skillet over a low heat until a light caramel is formed (this can take between 10 and 15 minutes). The caramel is best if you gently slew the sugar around the skillet repeatedly instead of stirring. Make sure that the caramel is not too dark, otherwise the crème will be too bitter. When the caramel has adopted a light brown color, you can add the butter and mix everything together until it develops a creamy consistency. Now's the time to add the “Fleur de sel”.