Use a food processor or a rolling pin to crush the digestive biscuits. Add the cocoa and mix well. Stir in the melted butter. Divide the crumbs between the serving glasses. Press the crumbs down gently and refrigerate while you make the mousse.
Melt the chocolate and set aside to cool slightly. Whisk the egg whites to stiff peaks and put to one side. In a separate bowl whisk the egg yolks, sugar and kahlua together until they have thickened slightly, about 4 minutes.
Add the egg yolk mixture and cream to the cooled chocolate and whisk until smooth. Gently fold in the egg whites until well combined.
Spoon the chocolate mousse into the glasses and chill in the fridge for 1-2 hours. Top with fresh raspberries just before serving.