Stir together first 6 ingredients in a 3 1/2-quart saucepan over medium-low heat until chocolate is melted and mixture is smooth.
Cook, stirring constantly, just until mixture comes to a boil. Reduce heat; cook, stirring constantly, 4 minutes.
Stir in milk, and cook until thoroughly heated. Whisk mixture until foamy.
Pour into mugs; top with miniature marshmallows, if desired.