In a heavy bottomed saucepan, heat the water and sugar over medium heat. Make sure to use a saucepan that's a little bigger than what you think you will need. Stir constantly until the sugar has dissolved and the mixture starts to bubble just a little.
Increase the heat to high and bring to a boil, stop stirring completely. Let the mixture continue boiling until it turns an amber color, this could take anywhere from 4-12 minutes.
Remove from the heat and slowly whisk in the heavy cream. The mixture will bubble up quite a bit, so make sure to do this very carefully. Then, mix in the butter, vanilla, and salt one at a time.
Pour the caramel sauce into a dish and allow to cool completely, then cover tightly and store in the refrigerator. The caramel sauce will thicken up some once it's cooled and refrigerated.