Add water to a medium pot. Slowly stream in the sugar in an even layer. (This is to help dissolve the sugar without the need to stir). Turn on the heat to medium and let the dissolved sugar mixture cook undisturbed until it has come to a full boil. Let the mixture continue to cook, largely undisturbed. You can use a very clean whisk to gently stir around the mixture as it reduces to help it heat evenly. Cook until the sugar has caramelized, about 4-6 minutes total cooking time. You'll know it's caramelized enough when the sugar is a deep amber, and is fragrant.
Carefully and slowly, whisk in pieces of the butter, a few at a time. Once the butter is whisked in, slowly stream in the heavy cream. Carefully whisk it together, being mindful of the very hot bubbling mixture.
Once the cream is whisked in, bring the caramel back up to a boil, then shut off the heat.
Remove the caramel from the heat, stir in the salt and let it sit for 5 minutes in the pan to slightly cool. Then transfer it to a heat-safe container for storage, or use it as desired.
