Prepare an ice bath by filling a larger bowl with ice water.
To make the custard for the pudding, in a medium saucepan, combine the cream, milk and vanilla bean (if using) over medium heat. Heat to just below a simmer.
Meanwhile, in a large bowl, whisk together the eggs and sugar until blended. When the cream mixture is ready, slowly pour it into the egg mixture while whisking continuously, and then continue to whisk until fully combined. Remove the vanilla bean (if used), and scrape the seeds from the pod into the bowl. Return the combined mixture to the saucepan.
Select a bowl that will be large enough to hold the custard and will rest in the rim of the bowl holding the ice bath. Place a fine-mesh strainer over the bowl. Place the pan over medium heat and stir the custard constantly with a wooden spoon until it is thick enough to coat the back of the spoon and steam just begins to rise from the surface, about 3 minutes. Immediately pour the custard through the strainer. Place the bowl over the ice bath and stir from time to time until the mixture is thoroughly cooled. (If it is warm, it will melt the chocolate.) Stir in the vanilla extract (if using).
Butter a 9-by-13-inch baking pan. Cut the brioche into half-inch cubes.
Spread one-third of the bread cubes in the prepared pan, and sprinkle with one-third of the chocolate. Repeat the layering twice, ending with the chocolate. Slowly pour the custard evenly over the bread, allowing it to settle and soak into the bread as you pour. Set the pan aside at room temperature for 1 hour, or cover and refrigerate overnight.
Position a rack in the lower third of the oven, and preheat the oven to 325°F.
Place the pan holding the pudding in a larger pan, such as a roasting pan. Place the nested pans on the oven rack, and pour the hottest tap water possible into the larger pan until it reaches about two-thirds of the way up the sides of the smaller pan. Bake the pudding for about 1 hour. Begin checking to see if it is ready after 50 minutes: press on the bread cubes in the center of the pan. If you see liquid rise around the bread, continue to bake the pudding. When the center is just set, carefully remove the pudding, still in its water bath. Let the bread pudding cool completely in the warm water, or for at least 30 minutes, before serving. Cut into desired shapes to serve.
NOTE: You can make it with leftover brioche, croissants, or even pain au chocolat. You can also add ¾ cup golden raisins instead of, or in addition to, the chocolate. Plump them first, if you like, by covering them with boiling water, letting them soak for a few minutes, and then draining, before layering them with the bread cubes.
