For the port caramel sauce:
Preheat oven to 325ºF.
Put 1.5 ounces bittersweet chocolate in a small heat-proof bowl. Heat 1/3 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 five minutes. Stir until the ganache is smooth. Let stand at room temperature until set.
In a small saucepan, melt butter and toss with the diced challah in a large bowl. Spread bread on a baking sheet in an even layer and toast for about 15 minutes, or until golden brown. Wipe the bowl clean.
Combine the remaining heavy cream with the milk and three tablespoons of the sugar in a small saucepan and bring just to a boil. Remove from heat. Add remaining bittersweet chocolate and let stand for five minutes. Whisk until chocolate is melted.
In the bowl used for the bread, whisk remaining three tablespoons of sugar with 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 toasted bread and toss to coat with warm chocolate custard. Let stand for 10 minutes, or until most of the chocolate custard is absorbed.
Pour bread mixture into prepared 6x6-inch baking dish. Using a large spoon, dollop the ganache on top. Bake the bread pudding for about 35 minutes, or until a toothpick inserted into the middle comes out clean.
Let the bread pudding stand for 15 minutes, then serve with the port caramel sauce on the side. This dessert can be made ahead and refrigerated over night. Rewarm before serving.
For the port caramel sauce:
Heat granulated sugar in a heavy medium saucepan over moderately high heat, stirring occasionally, until melted. Continue to cook, without stirring, until an amber caramel forms, about three minutes.
Remove from heat. Add light brown sugar and stir until smooth. Return caramel to the heat and carefully add the port. The caramel will harden slightly. Cook, stirring, until the sugar dissolves. Add cream and cook, stirring occasionally, until thickened, about six minutes. Remove from heat and stir in the vanilla. Transfer sauce to a pitcher and serve warm.
This sauce can be made ahead and refrigerated for up to three weeks. Rewarm before serving.
