In a medium, heavy saucepan, combine the sugar and water and bring to a boil over mediumhigh heat, stirring just until the sugar is moistened. Let the mixture boil, without stirring but swirling the pan occasionally, until it is a deep amber, smells like caramel, and you can see the tiniest wisps of smoke, 9 to 12 minutes. The caramel will be very hot. Remove the saucepan from the heat and carefully add a little bit of the cream; the caramel will bubble up furiously.
Place the pan over low heat and whisk in the remaining cream a little at a time (to avoid bubbling over). Then whisk in the butter, salt and vanilla. Continue to whisk until the sauce is very smooth, about 1 minute. Remove the pan from the heat and let the sauce cool; it will thicken as it cools. Serve warm or at room temperature.
Refrigerate in an airtight container for up to 3 weeks or freeze in a lock-top freezer bag for up to 2 months.
Yield 1 1/2 cups (360 ml)