Pour boiling water from the kettle into a heatproof mug to warm it up. Heat the milk and cocoa in a small pan, or in blasts in the microwave, until steaming. Discard the hot water in the mug, and tip in the chocolate egg pieces. Pour over the hot milk. Stir for 30 seconds or until the chocolate has melted. Top with the cream, sprinkle over the mini eggs or broken egg pieces and marshmallows to serve.