1.
Heat the cream almost to a simmer in a heavy medium-size saucepan. Remove the cream from the bowl, and divide it among three mixing bowls in these amounts: 1-1/4 c, 1 c., 3/4 c.
2.
Stir the bittersweet chocolate into the 1-3/4 c. cream, whisking until smooth. Stir the milk chocolate into the 1 c. cream, whisking until smooth. Stir the white chocolate into the 3/4 c. cream, and add the seeds from the vanilla bean; stir until smooth. Cover the three bowls loosely and refrigerate until the mixtures are thick but still pourable, 3 hours.
3.
Line a 8-1/2 x 4-1/2 in. loaf pan with aluminum foil. Leave some foil above the edge of the pan.
4.
Transfer the white chocolate mixture to a mixer bowl, and beat until it forms soft peaks--be careful not to overbeat it. Beat the last two or three strokes by hand; the mixture should be thick and firm. Scrape it into the prepared pan, smooth the top, and chill it in the freezer for 5 minutes.
5.
Repeat the beating process with the milk chocolate mixture and spread it over the white chocolate mixture in the loaf pan. Smooth the top, and freeze for 5 minutes.
6.
Repeat again with the bittersweet chocolate. Spread it over the milk chocolate mixture, smooth the top, and cover with plastic wrap. Refrigerate until firm, 3 hours.
7.
Lift up the foil to unmold the terrine. Remove the foil and cut the terrine into 1/2-in. thick slices. Spoon a pool of chocolate sauce onto each plate, and arrange a slice on top. Let stand 5 minutes before serving.
