Put the dark chocolate, marshmallows, butter, and boiling water in a saucepan. Heat over medium-low heat until melted and smooth, stirring occasionally. Remove from heat and allow to cool to room temperature.
Meanwhile, whip the whipping cream with vanilla extract until it holds its shape. Fold it into the chocolate mixture until you have a smooth mixture. Spoon the chocolate mousse into a serving bowl or individual dishes. Refrigerate for at least 6 hours or overnight before serving.