In a heatproof mixing bowl add your chocolate, 250ml of cream and your finely chopped marshmallows.
Place the heatproof bowl over a pot of simmering water.
Keep stirring until the mixture is melted and smooth, usually about 10 minutes.
Remove from heat and allow to cool to room temperature.
Beat remaining cream to soft peaks and fold through chocolate mixture.
Pour into glasses or bowls and refrigerate for at least 6 hours or overnight before serving.