1. Stir together the first 5 ingredients in a medium saucepan with 1 cup water. Simmer, stirring occasionally, until reduced to 4-1/4 cups, about 15 minutes. Chill, loosely covered, until cold, about 4 hours. Filling will thicken as it cools.Make vanilla custard::
2. Heat half-and-half in a medium saucepan over moderate heat just until hot (do not boil).
3. Whisk together yolks, sugar, cornstarch, vanilla seeds, and salt in a medium bowl. Gradually whisk in 1 cup hot half-and-half in a slow stream. Whisk egg mixture into remaining half-and-half in saucepan. Cook over low heat, stirring constantly with a wooden spoon, until thickened (do not boil) and custard registers 170 degree F on an instant-read thermometer, about 20 minutes. Immediately strain through a fine mesh sieve over a bowl. Cover surface of custard with plastic wrap and chill until cold and thickened, about 4 hours.Make port syrup::
4. Combine port, sugar, and 1/4 cup water in a small saucepan; simmer, stirring occasionally, until sugar is dissolved, about 3 minutes. Let stand until cooled to room temperature.To assemble trifle::
5. Cut each cake horizontally into about 2/3-inch-thick slices. Brush both sides of each slice evenly with port syrup and cut each slice crosswise into about 1-3/4-inch squares. Line the bottom of a 3-quart glass trifle or souffle dish with 1/3 of cake, overlapping if necessary. Top with 1/3 of berry mixture, 1/4 of custard, and 1/3 of chopped chocolate. Repeat layering 2 more times. Spoon whipped cream over top and garnish with chocolate shavings. Chill trifle and remaining custard at least 2 hours or overnight. Spoon trifle into bowls and serve with remaining custard on side. Makes 10 servings.