Prepare an ice-water bath. Heat sugar and the water in a small saucepan over medium-high heat until mixture boils and sugar dissolves, washing down sides of pan often with a wet pastry brush to prevent crystals from forming. Reduce heat to medium, and cook until sugar turns dark amber, 5 to 7 minutes more. Immediately remove from heat, and carefully whisk in 1/4 cup cream. Return to medium heat, and cook until sugar melts completely and mixture boils.
Remove from heat, and pour into a bowl set in the ice-water bath. Let caramel cool, stirring often, for 10 minutes. Stir in creme fraiche and salt. Cover and refrigerate at least 2 hours or up to 5 days.
Just before using, bring caramel sauce to room temperature. Whisk 1/4 cup cream until stiff peaks form. Gently fold the cream into the caramel sauce, using a rubber spatula, until incorporated.
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