Finely chop the chocolate and melt it over a double boiler and keep warm. Whip the cream until soft peaks form. Make a sabayon by combining the egg yolks, sugar and vanilla bean. Cook over a double boiler, whisking continuously until the egg yolks become thick and glossy, then remove the sabayon from the heat. Have the whipped cream nearby. Pour all of the melted chocolate into the warm sabayon and, with a large hand whisk, beat as fast as you can to incorporate all the chocolate into the sabayon. The chocolate will begin to seize; at that point you will need to add 1 large kitchen spoon of whipped cream. Continue to whisk; the cream will begin to soften the mixture. Add another large spoonful of cream and whisk vigorously for a further 1 minute. The chocolate, egg and cream mixture will now be more workable and relatively stable. You should have half the whipped cream left.
Immediately whisk the egg whites until soft peaks form. Place the whisked egg whites and the remaining cream on top of the chocolate mixture and gently fold through until well incorporated. Place the chocolate mousse in the refrigerator for a minimum of 6 hours before assembling the dessert.
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