In a medium heatproof 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.

In a small saucepan,bring the milk and cream just to a simmer. Add the chocolate mixture and whisk over moderate heat until hot. Remove the saucepan from the heat and add the whiskey. Pour the hot chocolate into a mug,garnish with marshmallow cream and serve.