Melt the chocolate in the top of a double boilder over hot but no boiling water. Beat the egg yolks lightly then stir into the melted chocolate. Heat the 2 minutes stirring constantly. Add orange rind and sugar; stir to combine well.
Set aside to cool slightly.
Meanwhile, whip egg whites with an electric mixer until stiff peaks form and then fold into the chocolate mixture until smooth. Pour into 6 small custard cups and chill several hours. Combine whipping cream, icing sugar and vanilla in a large metal bowl and beat with electric mixer until soft peaks form.
Top the chilled mousse with the whipping cream and garnish with fresh orange slices.