In a medium sized saucepan, gently heat together the milk, chocolate and brownie mix until the chocolate has melted completed and all the ingredients are fully combined.
Make a cornflour slurry by combining the 2 tablespoons of milk with the cornstarch. Slowly pour this slurry into the saucepan while continuously whisking.
When the hot chocolate has thickened to desired consistency, take off heat and pour into mugs.
Top with marshmallows and brownie bits if desired.