Before starting to prepare the mousse, refrigerate the heavy cream for 6 to 8 hours on the top rack of the fridge.
In a large mixing bowl whip ⅔ cup (about 150 ml) heavy cream until very thick.
In a medium heatproof bowl combine while chocolate, broken into pieces and ⅓ cup (50 ml) heavy cream. Place the bowl over a pan of gently simmering water. Stir until the chocolate melts. Let cool for 5 minutes, while whisking gently.
With a rubber spatula carefully fold the chocolate mixture into the whipped cream until homogeneous. Spoon the prepared white chocolate mousse into glasses or ramequins, filling them about ⅓. Set aside.
Whip another ⅔ cup (about 150 ml) until very thick.
Following the above process, melt also the dark chocolate together with the remaining ⅓ cup (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 the prepared sauce on top of the chocolate mousse.
Refrigerate the mouse for 2-3 hours before serving it.
You can place the bowl and the beaters of the mixer for 15 minutes in the freezer before starting to whip the cream, in order to ensure best results.
When combining the chocolate with the whipped cream, try first adding some of the cream to the chocolate mixture in order to temper it. Add some more, mix well and then add the whole chocolate mixture into the whipped cream.
Optionally you could prepare the raspberry sauce right before serving and pour it hot onto the refrigerated mousse.