Add everything to the slow cooker. Set the slow cooker to low. Stir to coat the chocolate chips with the sweetened condensed milk.
Turn the slow cooker to low. You will need to stir every 20-30 minutes, you do not want to scorch the chocolate. This will take about 1.5 hours to melt.
Serve over ice cream.
Notes
If you only have unsalted butter you can use that, just add a pinch of salt to the sauce.
If desired you can add a touch of vanilla or peppermint extract to the fudge sauce.
Store hot fudge sauce in an airtight container and store in the fridge for up to a week. You'll need to heat it up over medium heat on the stove top, stirring constantly. Remove from heat so it doesn't continue to cook. If it gets too hot, let the fudge sauce cool before serving.