Melt the chocolate, marshmallows and 125ml double cream in a large heatproof bowl set over a pan of gently simmering water (make sure the base of the bowl does not touch the water). Stir the mixture occasionally, until smooth (this takes a while). Leave to cool at room temperature, about 1hr.
Once mixture is cool, whip remaining cream in bowl until soft peaks form; transfer 125ml to small bowl for topping. Refrigerate.
Stir half of remaining whipped cream into cooled chocolate mixture until fully incorporated, then gently fold in the rest. Spoon into eight ramekins. Refrigerate until set, about 1hr.
To serve, top with reserved whipped cream and sprinkle with shaved chocolate, if you like.
