1) Combine the 2 chocolates, orange liqueur, 60ml water, and the vanilla in a heat-proof bowl. Set it over a pan of simmering water just until the chocolate melts. Cool completely to room temperature. Whisk in the orange zest and butter until combined.
2) Place the egg yolks and 100g of the sugar in the bowl of an electric mixer fitted with the paddle attachment. Beat on high speed for 4 minutes, or until very thick and pale yellow. With the mixer on low speed, add the chocolate mixture. transfer to a large bowl.
3) Place 240g of egg whites (save or discard the rest), the salt, and 1 tbsp of the sugar in the bowl of an electric mixer fitted with the whisk attachment. Beat on high speed until firm but not dry. Whisk 1/4 of the egg whites into the chocolate mixture; then fold the rest in carefully with a rubber spatula.
4) Without cleaning the bowl or whisk, whip the double cream and the remaining tablespoon of sugar until firm. Fold the whipped cream into the chocolate mixture. Pour the mousse into individual dishes or 2L serving bowl. Chill and decorate with whipped cream and oranges. Serve with extra whipped cream on the side.
For the whipped cream:
Whip the cream in the bowl of an electric mixer fitted with the whisk attachment. When it starts to thicken, add the sugar and vanilla and continue to whip until the cream forms stiff peaks. Don't overbeat, or you'll end up with butter.