Put 150 grams of mini marshmallows, butter, chocolate and water in a heavy-based saucepan, over a gentle low heat, stirring until melted and combined. Set aside to cool a little.
Whip the cream with the vanilla until thick soft peaks form then fold it into the chocolate mixture.
Divide between small glasses or ramekins and top with the remaining marshmallows and toast lightly with a blowtorch.