In your crock pot, add 12 oz. milk chocolate chips, 3 cups milk, 1 cup heavy whipping cream, 12 oz. evaporated milk, and 1 tsp. vanilla extract. Stir the mixture well to ensure all the ingredients are combined.
Set your crock pot to low heat. Allow the hot chocolate mixture to heat for 3 hours, stirring every hour to help the chocolate chips melt evenly and prevent burning.
Once the hot chocolate is hot and the chocolate has fully melted, ladle the cocoa into mugs. If using a punch ladle for serving, tilt it slightly to ensure a smooth pour and avoid spills.
Top each serving with marshmallows and a dollop of whipped cream for an added touch of sweetness.