Make ahead: Place the prunes in a bowl and pour the Cognac (or brandy) over them. Cover and let soak for at least 2 hours, or as long as overnight., Pour the heavy cream into a medium saucepan. Split the vanilla bean lengthwise and use the tip of a knife to scrape out the seeds. Add the vanilla seeds and pod to the cream and heat the pan over medium heat until small bubbles begin to form at the edge (don't let the mixture boil)., Remove the cream from the heat, cover, and let stand for 30 minutes at room temperature. Remove the vanilla bean pod, rinse, dry, and set aside for another use., To make the pudding: Spread the cubed bread in a buttered 9" x 13" pan., In a large mixing bowl, beat together the eggs, sugar, and salt. Add the cooled cream and vanilla extract and mix to combine., Pour the mixture through a fine-mesh strainer over the cubed bread. Gently nudge the bread down into the liquid., Drain the prunes. Sprinkle the prunes and chopped chocolate over the top of the custard and use the back of a spoon to press them under the surface., Refrigerate the bread pudding for 30 minutes while you preheat the oven to 350°F. Bring a kettle of water to a boil., Set the pan of bread pudding into a larger roasting pan on the oven's rack and add enough hot water to the roasting pan to come halfway up the sides of the pan. Carefully slide the rack back into the oven and bake until the pudding is just set and the top begins to take on some color, about 35 minutes for a metal pan or up to 40 minutes in a ceramic or glass dish., Remove the pudding from the oven and let it cool on a rack. Just before serving, preheat the broiler and toast the top of the pudding (it should be about 3" below the heating element) until nicely browned, watching constantly., Remove from the oven and let cool for 5 minutes before serving warm., Store leftovers, covered, in the refrigerator for up to 5 days.