Beat mascarpone cheese with the vanilla until smooth. Grate half the chocolate and add to mixture.
Melt the remaining chocolate in the microwave with the cream, using short 20-second intervals on high.
Divide 1/2 of the mascarpone mixture between 4 small ramekin dishes. Divide 2/3 of the raspberries between the dishes, then cover with the melted chocolate mixture. Cover with the remaining mascarpone mixture and then raspberries. Sprinkle the top of each dish with caster sugar.
Place the dishes under a moderate grill for about 3 minutes, or till the sugar on the top has gone golden brown. Chill for at least 30 minutes before serving.