Before starting to prepare the mousse, refrigerate 300 ml of the heavy cream for 6 to 8 hours in the top rack of the fridge. For best results place also the bowl and the beaters of the mixer for 15 minutes in the freezer.
In a large mixing bowl whip 150 ml heavy cream until very thick.
In a medium heatproof bowl combine while chocolate, broken into pieces and 50 ml heavy cream (not refrigerated). Place the bowl over a pan of gently simmering water. Stir until the chocolate melts.
With a rubber spatula carefully fold the chocolate mixture into the whipped cream until homogeneous. Spoon the prepared white chocolate mousse into glasses and set aside.
Whip the rest of the cold heavy cream (150 ml) until very thick.
Following the above process, melt also the dark chocolate together with the remaining 50 ml heavy cream.
Carefully fold the dark chocolate mixture into the whipped cream. Spoon the dark mousse in the glasses over the white mousse.
In a small saucepan over medium heat combine raspberries and sugar. Whisk until the mixture starts to thicken: about 10-15 minutes.
Spoon or pour the prepared sauce on top of the chocolate mousse.
Refrigerate the mouse for 2-3 hours before serving it.
Optionally you could prepare the raspberry sauce right before serving and pour it hot onto the refrigerated mousse.