For each 12oz chocolate, prepare a 1-pt (500mL) loaf pan by lining it with parchment. [For broader slices of the finished terrine, double the recipe and use a 1-qt (1L) loaf pan.]
Chop the chocolate into small pieces. Melt the chocolate over warm water. Do not allow any water to get into the chocolate.
Separate the eggs.
Whip the egg whites until they form soft peaks. Set aside. (It is necessary to whip the egg whites before beginning to mix the chocolate because once you add the first liquid to the chocolate, you must continue the procedure without interruption.)
Add the orange liqueur to the chocolate and beat in. The chocolate will become very thick.
Beat the yolks into the chocolate, 1 or 2 at a time, until well blended.
Beat the egg whites into the chocolate mixture. Use a stiff whip to beat them in. Do not try to fold them in gently, as the mixture is too stiff.
To create the necessary smooth texture, force the mixture through a sieve to remove lumps of unblended chocolate or egg.
Pour some of the chocolate into the prepared pan(s) to fill them about halfway. Rap the pans on the bench sharply to remove air bubbles. Fill with the remaining chocolate, and again remove air bubbles.
Cover and refrigerate overnight.
Unmold onto a platter. Dust the top and sides lightly with cocoa powder. Slice about ¼in. (6mm) thick using a sharp, thin-bladed knife dipped into hot water and wiped dry before each slice. Serve small portions, as this dessert is very rich.
Substitute 3 oz (90 g) strong espresso for the orange liqueur.