Break chocolate into small bits, then melt it over simmering water. Add half of the cream then the marshmallows. Stir until smooth, then let it rest.
Crush the raspberries and sugar with a fork and spoon them into 4 small glasses or bowls.
Whip the remaining cream until you get soft peaks, then fold it gently into the cooled chocolate mixture.
Spoon the mousse on top of the crushed fruit then put the glasses in the fridge for about an hour.
Just before serving, garnish the bowls with fresh raspberries and/or chocolate shavings.