I made this once, as an ending to a dinner for 10 (it’s a big cake!). It was a lot of work (especially all those chocolate curls), but it was really impressive and quite good (though Bruce, who was around for all those hours I was baking, thought it wasn’t worth the work when I could have just made the Our #1 Favorite Chocolate Cake). Having said that, it met my criterion for success: All the women asked for the recipe! I found it in Gourmet, Le Jour et La Nuit Hotel de Crillon, years ago, when I was Editor of Elle and was actually going to the Crillon a lot during the Paris spring and fall collections.
9 large egg yolks
1 cup sugar
6 T cornstarch
3 cups milk
1 T vanilla
14 oz. fine-quality bittersweet chcolate, chopped
4 cups well chilled heavy cream
14 oz. fine-quality white chocolate, chopped
For the garnish
6 oz. fine-quality white chocolate at room temperature
4 oz. fine quality bittersweet chocolate at room temperature
In a large bowl whisk together the egg yolks and the sugar until the mixture is combined well, add the cornstarch, sifted, and whisk the mixture until it is just combined. Whisk in the milk, scalded, in a slow stream, transfer the mixture to a heavy saucepan, and bring it to a boil, whisking constantly. Boil the pastry cream, whisking, for 1 to 2 minutes, or until it is very thick and smooth, strain it through a fine sieve into a heatproof bowl, and stir in the vanilla. Lay a piece of plastic wrap on the pastry cream and chill for 1 to 2 hours, or until it is cooled completely.
In the top of a double boiler or a metal bowl set over barely simmering water melt the bittersweet chocolate, stirring occasionally, and let it cool until it is likewarm. Transfer half the pastry cream to a large bowl, whisk in the chocolate, and whisk the mixture until it is combined well. In a chilled bowl with an electric mixer beat 2 cups of the heavy cream until it holds soft peaks, stir one fourth of it into the choclate mixture, and fold in the remaining whipped cream gently but thoroughly. Pour the dark chocolate mousse into a lightly oiled 10-inch springform pan, at least 2 1/2 inches deep, spread it evenly, and chill it while making the white chocolate mousse.
In the top of a double boiler or a metal bowl set over barely simmering water, melt the white chocolate, stirring occasionally, let it cool for 2 to 3 minutes, or until it is lukewarm, and whisk it into the remaining pastry cream until the mixture is combined well. In a chilled bowl with an electric mixer beat the remaining 2 cups heavy cream until it holds soft peaks, stir one fourth of it into the white chocolate mixture, and fold in the remaining whipped cream gently but thoroughly. Pour the white chocolate mousse over the chilled dark chocolate mousse, spread it evenly, and chill the cake, covered loosely with parchment or wax paper, overnight.
Prepare the garnish: Grate fine 2 ounces of the white chocolate and reserve it. Working over a foil-lined baking sheet or tray, shave curls from the remaining 4 ounces of white chocolate and from the bittersweet chocolate with a vegetable peeler, keeping the white and dark chocolate curls separate, and chill them, covered loosely, for a least 1 hour, or overnight.
Run a thin knife around the edge of the cake, remove the side of the pan, and smooth the side of the cake with a spatula. Put the cake on a rack set in a shallow baking pan, press the grated white chocolate onto the side, gathering the excess in the pan and reapplying it until the side is coated evenly, and transfer the cake to a platter. Mound the white chocolate curls carefully on half the cake and the dark chocolate curls on the other half and chill the cake, if desired, for up to 4 hours.