In a saucepan, combine the sweetened condensed milk and chocolate chips. Heat over medium heat, stirring constantly until the chocolate chips are completely melted.
Remove the saucepan from the heat and stir in the unsalted butter and vanilla extract. Keep stirring until the butter is fully melted and incorporated.
Transfer the hot fudge sauce into a jar. It's ready to use while still warm.
Any leftovers can be stored in the refrigerator for up to 2 weeks. To reheat, place the jar in the microwave and heat in 30-second intervals, stirring between each, until the sauce is melted and smooth.
Notes
For a richer flavor, you can add a pinch of salt to enhance the sweetness of the hot fudge sauce.