4.

To make the caramel filling: Place 4 ounces (1/2 cup) of water in the bottom of a large saucepan. Add the sugar and corn syrup and cook the mixture over medium-high heat. Wash down the sides of the pot using a pastry brush dipped in cool water. The mixture will begin to turn amber and smoke a bit, at which point, you should swirl the contents to even out the mixture. After about 10 minutes, you will have a dark amber caramel. Remove it from the heat, add the butter, cream, creme fraiche, and salt (be careful as you do, the caramel mixture will sputter), and whisk until the caramel is smooth. Pour the caramel out into the cooled shell. Let the tart sit for 1 hour, until the caramel is cool.