Submerge the gelatin leaves in a bowl of cold water to soften. Heat the orange juice until it is hot but not boiling then squeeze the excess water out of the gelatin and stir it into the orange juice until it is fully dissolved. Set aside to cool slightly.
Place the egg whites in a spotlessly clean bowl and whisk with an electric mixer until they just form stiff peaks; set aside. In a separate bowl, whisk the cream until it forms soft peaks (there is no need to wash the beaters as long as you whisk the egg whites first); set aside.
Put the egg yolks, sugar and orange zest into a bowl and whisk with an electric mixer (again, no need to wash the beaters) over a pan of gently simmering water for 5-10 minutes until thick and foamy. Remove from the heat and continue whisking until completely cool.
Gently fold the orange juice gelatin mixture into the egg yolks followed by the egg whites and cream. Divide the mixture between six large glasses or eight small ones and place in the fridge for a couple of hours to set.
Place the chocolate, butter and water in a heatproof bowl and set it over a pan of very gently simmering water (make sure that the base of the bowl doesn't touch the water). Stir until the chocolate has melted then remove from the heat.
Allow the chocolate to cool for a couple of minutes then stir in the egg yolks one at a time. (The butter and water should prevent the chocolate from seizing when you add the yolks but if it does happen then you can rescue it by stirring in a little boiling water, a Tbsp at a time, until the chocolate becomes smooth again.).
In a clean bowl, whisk the egg whites and caster sugar with an electric mixer until it just forms stiff peaks. Then in a separate bowl, whisk the cream until it forms soft peaks (there is no need to wash the beaters first).
Fold a spoonful of the egg whites into the chocolate to loosen it then gently fold in the rest of the whites and the cream.
Divide the mousse between the glasses on top of the orange mousse, level the tops then return to the fridge for at least a couple of hours (or overnight) to set. The mousse pots will keep in the fridge for a couple of days.
Whisk the butter and sugar together until smooth. Add the cocoa powder and flour and stir in until the mixture resembles wet sand then stir in the chocolate chips.
Use your hands to bring the mixture together into a soft dough. Shape it into a rough log, about 5cm thick, then wrap it in clingfilm and twist the ends to seal. Roll it on the worktop a little to make the log smoother then refrigerate for at least a couple of hours, preferably overnight, and up to a couple of days.
Preheat the oven to 180C/350F/gas mark 4 and line a couple of baking sheets with baking parchment.
Use a very sharp knife to slice the log into rounds about 8mm thick and place them slightly spaced apart on the baking trays. The chocolate chips can make them a little difficult to slice, but any rounds that crumble can simply be pressed back together with your hands.
Bake for 10-12 minutes then allow to cool on the tray for 10 minutes before transferring to a wire rack to cool completely; they will firm up as they cool. It can be difficult to judge when the biscuits are cooked due to the cocoa powder so it may be best to do a test batch to see how long they need in your oven.