1. Combine cream and milk in a large saucepan, and cook until hot but not boiling. Keep warm.
2. Combine sugar and 1/2 cup water in a large, heavy saucepan. Bring to a boil over medium-high heat, and cook, swirling pan, about 2 minutes or until sugar dissolves. Boil, undisturbed, 5 minutes or until caramel is a deep amber color. Remove from heat, and immediately add cream mixture to caramelized sugar. (Mixture will be very hot and will boil up.) If some of the caramelized sugar hardens on bottom of pan, whisk over medium-low heat until sugar melts and mixture is smooth.
3. Gradually whisk about 3 cups hot caramel cream into egg yolks; add yolk mixture to saucepan, and whisk until well blended. Place pan over low heat, cooking until a thermometer registers 160°. Set pan in a large, ice-filled bowl until completely cooled, stirring occasionally. Stir in vanilla and fleur de sel.
4. Pour mixture into an ice-cream maker, and freeze according to manufacturer's instructions. Sprinkle each serving with additional fleur de sel, if desired.