Measure out all of your ingredients ahead of time, and set them aside. The caramel comes together very quickly, so you won't have time to measure in between steps.
In a large, heavy bottomed saucepan, combine water then sugar. Place the pot over medium heat and cook, without stirring, until the sugar dissolves, thickens, and turns a deep amber color. If needed, gently swirl the pan every so often to ensure even browning. Keep a constant eye on the sugar - it will brown suddenly!The mixture will bubble a lot and turn foggy in the early stage, then the bubbling will calm down and the mixture will begin to caramelize. Take care not to let the sugar get too dark or it'll taste burnt!* If you notice hardened/crystallized sugar along the edges of the pot, brush the sides with cool water to help dissolve it, or cover your pot with a lid until dissolved.
As soon as the sugar has turned an amber color, quickly (but carefully!!) add in your butter. Cook, mixing constantly with a long handled wooden spoon, until the butter is fully melted. The mixture will bubble and almost triple in volume, but once the butter is mixed in it should settle down.
Pour in your heavy cream and add your salt to the pot. The mixture may seize up because of the cold cream, but continue cooking and stirring until it all melds together. Boil the mixture for 1 minute without stirring, then take off the heat and transfer the caramel sauce immediately to a heat safe jar. Cool to a safe to handle temperature before serving!