In a Dutch oven (or heavy bottom saucepan) combine sweetened condensed milk and chocolate chips over medium high heat.
Stir constantly until chocolate is completely smooth.
Add heavy cream, milk, and vanilla.
Whisk until fully combined. Serve warm with added marshmallows or whip cream. Enjoy!
Notes
This can be done in a crock pot. Add all ingredients to crock pot and cook on low 3-4 hours or high 1-2 hours before serving. Be sure to stir every hour