Preheat the oven to 325°. Put 2 ounces of the bittersweet chocolate in a small heatproof bowl. Heat 1/4 cup of the heavy cream in a small saucepan over moderately low heat. Pour the warm cream over the chopped chocolate and let stand for 5 minutes, then stir until the chocolate ganache is smooth. Let the chocolate ganache stand at room temperature until set.
Butter an 11-by-8-inch baking dish. In a small saucepan, melt the butter. In a large bowl, toss the melted butter with the diced challah. Spread the bread on a baking sheet in an even layer and toast for about 15 minutes, or until the bread is golden brown. Wipe out the bowl.
In a small saucepan, combine the remaining 1 cup of heavy cream with the milk and 6 tablespoons of the sugar and bring just to a boil. Remove the heavy cream mixture from the heat. Add the remaining 3 ounces of chopped bittersweet chocolate and let stand for 5 minutes, then whisk until the chocolate is melted.
In the bowl used for the bread, whisk the remaining 6 tablespoons of sugar with the egg yolks, cocoa, vanilla and salt until a paste forms. Slowly whisk in the warm chocolate cream until smooth. Strain the custard into a clean bowl. Add the toasted bread and toss to coat with the warm chocolate cream. Let stand for 10 minutes, or until most of the chocolate cream is absorbed.
Pour the bread mixture into the prepared baking dish. Using a large spoon, dollop the chocolate ganache on top. Bake the bread pudding for about 35 minutes, or until it is cooked through. Let the chocolate bread pudding stand for 15 minutes, then serve with the Port Caramel Sauce on the side.
The chocolate bread pudding can be refrigerated overnight. Rewarm before serving.
A vintage character port will have the sweetness to stand up to the caramel and the rich, plummy flavors here.