Combine sugar and water in a heavy bottomed saucepan. Stir together, then run a rubber spatula around the inside edge of the pan to remove any bits of sugar. This will prevent crystallization.
Place the saucepan over low heat until the sugar starts to dissolve. Turn the heat up to medium-high until it starts to thicken. DO NOT STIR. Watching the pot is highly recommended. Caramel burns very easily. It shoud be a deep amber in color and smell slightly toasted.
Add the butter carefully. The mixture will roil and bubble. Whisk until all of the butter is blended. Turn off the heat. Add the heavy cream very slowly - again, the mixture will roil and bubble.
Add the vanilla and salt and stir until smooth. Remove from heat and allow to cool. To come to room temp will take about 3-4 hours. The sauce will thicken again as it cools.
Keeps for about two weeks in an air-tight container in the refrigerator. It won't last that long, though!