Liberally butter a 9x13" baking dish. Set aside.
In a large pot with a candy thermometer attached to the side, set over medium heat, combine the sugar, syrup, and salt. Stir the mixture until the syrup's dissolved & begun to bubble.
Without letting the mixture stop bubbling, slowly whisk in the cream until it's completely incorporated. Don't stop stirring.
Stir in the butter, and keep stirring. The mixture must remain at a boil, stirring constantly, for roughly 15-20 minutes.
When the temperature reaches 245° stir in the vanilla, then carefully pour the mixture into the prepared dish.
Set the dish aside, and allow to cool completely so the caramel hardens.
Once hardened, use a butter knife to cut it into rectangular or square pieces.
Wrap the pieces in wax paper, and they will stay good (and soft) for 2-3 weeks.