- Combine the milk, heavy cream, sugar, cocoa powder, and a pinch of salt in a medium saucepan.
- Cook over medium heat, whisking occasionally, until the sugar and cocoa powder dissolve and the milk is steaming; do not boil.
- Whisk in half of the chopped chocolate until melted, then whisk in the remaining chocolate until smooth.
- Remove from the heat and whisk in the vanilla. Pour into mugs and top with marshmallows or whipped cream.