If you can’t get salted butter, most salted butter has about ¼ teaspoon of salt per 4 ounces (115g), so you can use that as your guide, and use unsalted butter.
Serve the sauce warm with ice cream, or cool it to room temperature and store it in the refrigerator for 1 to 2 weeks. Rewarm the sauce by putting the jar in a pan of very hot water.