Place the marshmallows, milk, butter and salt into a saucepan and heat over a low heat until melted, stirring regularly.
Add in the chocolate chips and vanilla extract and continue to cook whilst stirring until the chocolate has melted and you have a super smooth and glossy sauce.
Serve whilst warm.
This can be stored in an airtight container in the refrigerator for up to two weeks.
Notes
Be cautious when adding the butter— it will bubble up!
You can store it for up to two weeks, but it never lasts that long in our house.
This sauce is also perfect for dipping churros or doughnuts.