Elizabeth David gives a basic chocolate mousse recipe in French Provincial Cooking, but with the addition of the juice of an orange. I tried it at the weekend. Forgetting my previous recipe with two egg yolks and four whites, I used four yolks; the only other change to the procedure was that I added the orange juice to the melted chocolate and butter before stirring in the egg yolks, and folding in the whites.
Verdict: so-so. The mousse could not be anything other than delicious, of course; but it did not hold in suspension the juice, which collected in the bottoms of the ramekins. Instead, I should have used orange zest.