Dissolve the cocoa

• Warm a cup of the milk in a saucepan.
• Whisk in the cocoa powder until completely smooth.

Fill the slow cooker

• Pour the cocoa mixture into the slow cooker.
• Add the remaining milk, heavy cream, sweetened condensed milk, chocolate chips and vanilla.
• Stir everything together well and cover with the lid.

Cook on low

• Cook on LOW, stirring occasionally, until the chocolate is fully melted and the mixture is smooth and creamy.

Serve

• Pour into mugs and top with marshmallows.