In a small saucepan, whisk the cocoa powder, sugar and salt with about 1/4 cup of the milk to form a smooth paste.
Set the pan over medium-low heat and gradually whisk in the rest of the milk.
Heat, whisking frequently, until the milk is steaming and small bubbles form around the edge, about 5 minutes. Do not let it boil.
Remove from the heat and add the chopped chocolate. Let stand 30 seconds, then whisk until melted and smooth.
Stir in the vanilla, taste and add a little more sugar if you like.
Pour into mugs and top with whipped cream or marshmallows if desired.