In large bowl, combine chocolate, marshmallows and ½ cup heavy cream. Microwave on High 2 minutes; stir until marshmallows are melted and mixture is smooth (if not melted after 2 minutes, microwave in 15-second intervals, stirring in between). Let cool to room temp, about 45 minutes.
Once mixture is cool, whip remaining cream in bowl until soft peaks form; transfer ½ cup to small bowl for topping. Refrigerate.
Stir half of remaining whipped cream into cooled chocolate mixture until fully incorporated, then gently fold in the rest. Spoon into eight 4-ounce ramekins. Refrigerate until set, at least 2 hours and up to overnight.
To serve, top with reserved whipped cream and sprinkle with shaved chocolate, if desired.
