Heat ½ cup water in a saucepan over medium-high heat. Add cocoa powder little by little, stirring continuously to make sure there aren't any lumps. Bring the mixture to the boil. Add milk and chocolate. Stir until chocolate is completely melted. Divide among serving cups.
Marshmallow Foam:
Place marshmallows in a microwave safe bowl. Microwave on high for 40-45 seconds or until marshmallow has puffed and risen.
Top hot chocolate with marshmallow foam. Serve immediately.