For the chocolate délice, place a 25cm/10in flan ring (6cm/2½in deep) onto the sponge base and press down to cut out a circle, leaving the sponge at the bottom of the flan ring. Drizzle orange liqueur over the sponge base.
Heat the milk and cream in a saucepan over a medium heat until it just comes to the boil. Meanwhile, whisk the eggs in a bowl. When the milk and cream mixture is just boiling, pour it over the beaten eggs, whisking the mixture constantly until it forms a smooth, thick custard. Add the chopped chocolate and whisk again until it has melted and the custard is smooth.
Pour the chocolate custard into the flan ring on top of the sponge base. Gently tap the sides of the flan ring to release any trapped air bubbles, then smooth the surface of the custard using a palette knife. Cover with cling film and chill in the fridge for at least 6 hours, or ideally overnight, until completely set.
Meanwhile, if making the orange curd, whisk together the orange juice and zest, the lemon juice and zest, the sugar and the eggs and egg yolks in a saucepan, then heat over a medium heat, stirring constantly with a wooden spoon, until the mixture thickens enough to coat the back of a spoon (about 8-10 minutes). Reduce the heat to a low heat and continue to cook the mixture for a further 1-2 minutes, still stirring constantly.
Remove the pan from the heat and stir in the butter until melted. If the orange curd is not perfectly smooth, strain it through a fine sieve. Transfer the curd to sterilised jars, then seal and set aside to cool. Chill in the fridge until needed. (The orange curd will keep in the fridge for up to two weeks.)
To serve, dust the délice with the cocoa powder, then carefully remove the flan ring (you can warm the sides of the flan ring with a chefs’ blow torch to make this process easier, if you have one). Serve the délice in slices with a dollop of the orange curd alongside.