To make the caramel in a slow cooker, pour the condensed milk into a heatproof canning jar and cover with a watertight lid. Place jar into the slow cooker and fill with water to cover the jar. Cook on Low for 8 to 10 hours, making sure to add more water as necessary. Let caramel cool at room temperature before opening the jar.
To make a salted caramel frosting, add a heaping 1/2 teaspoon of table salt or 1 heaping teaspoon sea salt to the frosting.