In a bowl, using an electrical mixer, whip egg whites until foamy. Gradually add sugar until stiff peacks are formed.
Beat egg yolks with orange zest, then add to the chocolate mixture.
Fold in a spoonfull of egg whites into the chocolate mixture, then pour the chocolate mixture into the remaining egg whites. Fold very gently until combined.
Pour the mousse into glasses or ramequin dishes and place in the fridge for a few hours or overnight.
Garnish with raspberries and dust with icing sugar before serving, if desired.
*** to make the chocolate flower decoration: melt some chocolate in bain marie, pour it into a piping bag and "draw" flowers on parchment paper, leave in the fridge for an hour or two.