Put sugar in a large, heavy stock pot over medium heat. (The stock pot should have plenty of extra room.) Stir frequently. At first not much will happen, but as the sugar heats, it will begin to melt. The sugar will become sandy-looking and pebbly as melted chunks are surrounded by dry.
Eventually the sugar will melt. It proceeds quickly at this point. Take it as dark as you want it–it will develop a bit more as you finish it, but not much. The darker the sauce is the more caramel flavor but the less sweetness. Ideally all the sugar will be liquid, but it's ok if a few bits remain hard. They should dissolve when the butter gets added.
Put on an oven mitt over the hand you use to stir, as the next steps release a great deal of steam and you can get burned.
Add the butter a piece or two at a time and stir until incorporated. The mix will foam and increase in volume. Once the butter is incorporated and the steam has steamed off, the foaming will stop.
Add the cream slowly while stirring rapidly. If you add too fast or stir too slowly, the mixture will crash out and crystalize. (If that happens, you may be able to rescue it by persistent stirring over medium heat.)
Add the salt and stir.
This makes about 5 1/2 (8 oz) mason jars.
This batch size is as large as I'm comfortable working with, due to how hard it is to stir the sugar during the "pebbly" stage and due to the size of our stock pot vs how much the batch foams up.