Combine the sweetened condensed milk, ¼ cup water, and salt in a small saucepan, and set over low heat. Add the chocolate and stir to incorporate. Continue to cook, stirring slowly but constantly, just until the chocolate has melted and the mixture is shiny and smooth. At this point, you might want to add more water to loosen things up—add in 1-tablespoon increments until the consistency is sundae-worthy to you. Stir in a bonus ingredient (see the Author Notes for suggestions) if you want, and adjust the salt to taste.
Serve warm with ice cream (especially vanilla or coffee), frozen yogurt, Greek yogurt, pudding, you name it. Store any leftovers in the fridge.