Combine all ingredients except marshmallows in a slow cooker and stir to combine.
Cook, covered, until everything is melted, stirring occasionally, 2 hours on LOW.
Ten minutes before serving, add marshmallows and let them get melty.
Garnish with more mini marshmallows, and additional toppings crushed peppermint, chocolate shavings, chocolate syrup, whipped cream, caramel sauce, etc