Put the sugar in a large heavy saucepan over medium heat. Stir vigorously with a wooden spoon until the sugar melts, this will take a few minutes.
Once the sugar has all melted start adding the butter one piece at a time. Whisk until the butter is all melted
Wait about 3 seconds and then add the cream slowly. Whisk as you add until the mixture is smooth and creamy. Remove from heat.
Add the salt and whisk again a couple times just to incorporate.
Let cool in the pan for a bit and then pour, carefully, into a mason jar to store. It should keep for about 2 weeks in the fridge but I haven't tested that since it never lasts more than 24 hours in my house.
Recipe by Foods of Our Lives at https://www.foodsofourlives.com/2011/06/salted-caramel-sauce/