Put all of the ingredients into a slow cooker and stir to combine.
Cover and cook on low for 3-4 hours, stirring every 30-45 minutes to prevent the chocolate from burning on the bottom.
Once the chocolate is melted and the cocoa is hot, turn the heat down to the keep warm setting. If a film forms on the top, whisk the hot chocolate until smooth again.
Ladle into mug and top with marshmallow or whipped cream and chocolate shavings, if desired.