Put the marshmallows, butter, chocolate and 60ml/¼ cup of freshly boiled water in a heavy-based saucepan.Put the saucepan on the hob, over heat, though keep it fairly gentle, to melt the contents, stirring every now and again. Remove from the heat.While the chocolate mixture cools, whip the cream with the vanilla extract until thick, and then fold into the cooled chocolate mixture until you have a smooth, cohesive mixture.Pour or scrape into 4 glasses or ramekins, about 175ml / ¾ cup each in capacity, or 6 smaller (125ml / ½ cup) ones, and chill until you want to eat. The sooner the better!