Melt the chocolate, butter and syrup in a small heavy bottomed saucepan over low heat. Transfer to a medium bowl and set aside to cool slightly.

Combine the cream and sugar sub in a small bowl, whip until soft peaks form, fold one third of the cream into chocolate until just combined, fold in remaining cream. Garnish each serving with 1 tablespoon toasted chopped hazelnuts and a raspberry. Serve mousse right away or cover and refrigerate for up to 3 hours.