Set your freezer to rapid freeze at least 30 minutes before required and remember to return it to its normal setting afterwards.
Scoop out the flesh from the oranges, leaving the skins intact. If necessary, cut a small slice off the base of each orange half so that they stand upright. Put the orange and mango flesh, orange zest, yoghurt and crème fraîche in a blender. Blend for a few minutes until smooth. Spoon the mixture into a freezeable container and freeze for 2 hours, stirring occasionally to break up the ice crystals.
Spoon the mixture into the orange halves. Return to the freezer and leave for at least 1 hour, or until frozen solid. Let soften in the refrigerator for at least 30 minutes before serving. Serve 2 orange halves per person sprinkled with some freshly-grated chocolate.
Variation: You could use pink grapefruit instead of the oranges; add 2 tablespoons of finely grated zest to the yoghurt mixture. You could simply freeze the mixture in a container and serve it in scoops after letting it soften in the refrigerator.