Preheat oven to 325 degrees F. Butter a 3-quart rectangular baking dish. Spread bread cubes evenly in the prepared baking dish; set aside.
In a medium saucepan, combine milk, sugar, and chocolate. Cook over medium heat until chocolate is melted, whisking frequently. Remove from heat.
In a large bowl, combine eggs, orange peel, vanilla, and salt. Gradually whisk in chocolate mixture. Pour evenly over bread in dish. Press down lightly with a rubber spatula or the back of a large spoon to moisten all of the bread.
Bake, uncovered, for 45 to 50 minutes or until evenly puffed and set. Cool on a wire rack for 30 minutes. Serve warm. If desired, serve with whipped cream.