Place chocolate and butter in a heatproof bowl set over a saucepan of barely simmering water (don't let bowl touch water), stir chocolate till melted and then remove from heat and sir in orange zest, orange juices and egg yolk until combined.
Using an electric mixer, beat cream to soft peaks and then fold into chocolate.
Using an electric mixer, whisk egg whites until firm peaks form, add icing sugar and whisk until thick and glossy and then fold half the egg whites into chocolate mixture and then gently fold in the remainder.
Spoon mixture into 4 x 125ml (1/2 cup) ramekins and place in refrigerator for 3 hours or until set.
For citrus salad, peel and segment oranges. Cut some of the peel into thin strips of zest. Peel and segment the mandarine and then combine the fruits and the zest in a small bowl.
For the orange cream, using an electric mixer whisk cream and icing sugar until soft peaks form and then fold in zest.
Sserve mousse with the citrus salad and orange cream.