In a medium saucepan combine butter, chocolate, water, and marshmallows. Set over moderately low heat and cook, stirring frequently until mixture is melted and smooth. Set aside to cool until warm, 7 - 10 minutes (it may appear to separate a little just whisk vigorously to blend).
Meanwhile, whip heavy cream with vanilla until stiff peaks form. Fold and stir whipped cream into chocolate mixture. Spoon into 6 dessert cups. Chill 30 minutes or until ready to serve. If mousse chills through entirely after several hours of refrigeration, just allow it to rest at room temp about 20 minutes for optimal consistency. Top with whipped cream, raspberries and mint leaves if desired, or top with whipped cream and shaved chocolate if desired.