Step 1

Whisk together the sugar and cocoa powder in a 6-quart slow cooker. Gradually add the milk, whisking constantly to incorporate. Whisk in the heavy cream, chopped chocolate, vanilla extract, and instant espresso.

Step 2

Cover and cook on low heat, whisking occasionally, until the chocolate is melted and the mixture is hot and well combined, about 2 hours. Add the marshmallows and cook until the marshmallows begin to melt into the hot chocolate, about 2 minutes. Stir in the Irish cream liqueur, if using.

Step 3

Set the slow cooker to warm. Ladle the hot chocolate into serving mugs, and top with more marshmallows and shaved chocolate, if you like.