1. Put the marshmallows, butter, chocolate, and water in a heavy-bottomed saucepan.
2. Put the saucepan on the stove over fairly gentle heat to melt, stirring every now and again. Remove from heat.
3. Meanwhile, whip the cream with the vanilla until thick and then fold into the cooling chocolate mixture until you have a smooth, cohesive mixture.
4. Pour/scrape into 4 glasses or small dishes, about 3/4 cup each in capacity, or 6 smaller (1/2 cup) ones, and chill until you want to eat. The sooner the better!