Caramel Filling: 1. Stir sugar, ¼ cup water, and corn syrup in deep medium saucepan over low heat until sugar dissolves. Increase heat to medium; cover pan and cook 4 minutes. Uncover; increase heat to high. Boil without stirring until syrup is deep amber, occasionally brushing down pan sides with wet pastry brush, about 6 minutes. Remove from heat. 2. Add cream (mixture will bubble). Whisk in butter, then crème fraîche, lemon juice, and pinch of fleur de sel. Cool completely. Can be made up to 3 days in advance as long as it is covered and kept cool.
For Macarons: 1. Preheat oven to 350°F. Line 1 large cookie sheet with mat. 2. Finely grind hazelnuts in processor. Add sugar and flour; blend to powder. With machine running, add egg white and vanilla; blend to paste. 3. Spoon mixture into pastry bag fitted with round tip. Pipe ¾- to 1-inch-diameter mound onto parchment about ½ inch high and spacing 1 inch apart. 4. Dip finger into water and gently press top of cookie to flatten center slightly. 5. Bake until golden brown, about 12 minutes). Allow to cool for about 2 min before transferring to wire rack to cool.