Combine cream and milk in a large saucepan, and cook until hot but not boiling. Keep warm.
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.
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 16
Set pan in a large, ice-filled bowl until completely cooled, stirring occasionally. Stir in vanilla and fleur de sel.
Pour mixture into an ice-cream maker, and freeze according to manufacturer's instructions.
Sprinkle each serving with additional fleur de sel, if desired.