Combine the sweetened condensed milk, unsweetened cocoa and salt in a saucepan.
Heat over medium heat while stirring constantly until combined, then pour in the milk.
Continue stirring until smooth, and cook until the mixture thickens enough to coat the back of a spoon.
Remove the saucepan from the heat. Stir in the vanilla extract.
Serve warm or let cool, then store in a mason jar in the fridge for up to a month.
