This pudding can be served warm, room temperature, or cold. To chill, let cool completely, then cover with plastic wrap, and refrigerate at least one hour or overnight.
Preheat oven to 350 degrees. Lightly butter an 11-by-7-inch (or 9-inch square) baking dish; set aside. Toast bread (in the oven or toaster) until lightly crisped.
In a medium saucepan, combine milk, chocolate, and butter; place over medium-low heat, stirring occasionally, until chocolate has melted, about 5 minutes.
Tear bread into large pieces (about 4 or 5 per slice); scatter evenly in prepared baking dish. In a medium bowl, whisk together eggs, granulated sugar, and vanilla. Whisk in warm milk mixture until combined. Pour over bread.
Bake until pudding has puffed and is firm, about 25 minutes. Cool at least 10 minutes, and dust with confectioners' sugar just before serving.
Using a cinnamon-raisin bread adds even more flavor to this chocolatey pudding.