In a saucepan, melt the butter, marshmallows, chocolate, peppermint extract, and water together. Mix until fully incorporated. Set aside to slightly cool.
In another bowl, whip heavy cream and vanilla into stiff peaks. Pour the chocolate mixture into the whipped cream, and gently fold the ingredients together. Place mixture into a piping bag and swirl into cups. Place in fridge for at least 30 minutes.