Preheat oven to 325°F. Butter a 2 1/2- to 3-quart rectangular baking dish. Place bread cubes in prepared baking dish, spreading evenly. Set aside.
In a medium saucepan, combine milk, sugar, and chocolate. Cook over medium heat until the chocolate melts, whisking frequently. Remove from heat.
In a large bowl, combine eggs, orange peel, vanilla, and salt. Gradually whisk in the chocolate mixture. Pour mixture over bread in the baking dish. Press lightly with back of spoon to be sure all bread cubes are moistened.
Bake for 45 to 50 minutes or until evenly puffed and set. Cool for 30 minutes; serve warm. If desired, serve with whipped cream.
Prepare as directed through step 3. Cover and chill for up to 24 hours. Uncover and bake as directed in step 4.