Pour milk and cream into the slow cooker.
Whisk in cocoa powder and salt until no dry pockets remain.
Add sweetened condensed milk and chocolate chips; stir to combine.
Cover and cook on LOW for 2 hours, stirring every 30 minutes to melt the chocolate evenly.
Stir in vanilla. Taste and add sugar if you prefer it sweeter.
Switch to WARM for serving. Ladle into mugs and top with marshmallows.