1. In a medium bowl, combine the bittersweet chocolate with the milk chocolate. Add the boiling water and let stand for 1 minute. Whisk the chocolate mixture until it is smooth.
2. In a small saucepan, bring the milk and heavy cream just to a simmer. Add the chocolate mixture and whisk over moderate heat until hot. Remove the saucepan from the heat. Pour the hot chocolate into demitasse cups or small mugs, garnish with marshmallows and serve.