In small saucepan over medium heat, heat milk and half-and-half with vanilla bean until milk is almost to the boiling point. As it cooks, swirl in pan to help distribute flavors of vanilla. When milk starts to coat the bottom of the pan, you'll know its done.
Remove from heat and add chocolate. Stir until fully melted. Remove vanilla bean before serving.
Top with peppermint marshmallows, if desired, and roll them around in the chocolate to speed up the melting.