Pour boiling water from the kettle into a heatproof mug or glass to warm it up. Heat the milk in a small pan, or in blasts in the microwave, until steaming. Pour the hot water out the mug, and tip in the chocolate and vanilla. Pour over the hot milk and stir for 30 seconds until the chocolate has melted. Top with the cream, marshmallows and drizzle over the sauce with a pinch of sprinkles and cinnamon, if you like.