Carefully cut the top off 6 oranges and scoop out them out, scraping the sides. Save the juice.
Place the chocolate in a large bowl and heat over a pan of simmering water (taking care that the bowl doesn’t touch the water beneath) until nearly melted or pop in the microwave for 30 second bursts until nearly melted. Stir well each time you remove from the microwave after a 30 second burst.
Add the cream to a saucepan and place over gentle medium heat. Heat the cream to just a gentle simmer with small bubbles foaming at the edge but not boiling.
Add the warm cream to the semi-melted chocolate and stir until it’s smooth and glossy.
Allow to cool a little, then stir in the egg yolk, orange zest and orange juice.
Beat the whites with an electric whisk until they form stiff peaks. Add the sugar and beat again until glossy.
Using a metal spoon, carefully fold a large spoon of egg whites in to the chocolate mix, and beat together. This will loosen it. Then pour this chocolate mixture into the rest of the egg whites and fold together gently, avoiding knocking any air out of the whites.
Nestle the oranges into a baking dish so that they stay upright. Pour the mousse into the oranges. Or pour mousse into a large bowl and leave to set in the fridge. The oranges will need less time to set, around 4 hours to overnight whereas the large bowl will need at least 8 hours to overnight.