Preheat oven to 350 degrees. Butter a 13x9x2 baking dish.
Trim off the thick, dark edges on the pandoro and cut the remaining loaf into 1-inch cubes. Spread in a single layer on two baking sheets, and bake for 10-12 minutes until lightly browned and fragrant. Set aside to cool.
Toss bread cubes with raisins and chestnuts (if using) and place in prepared baking dish. In a large bowl, whisk together eggs, egg yolks, half and half, sugar, and vanilla. Slowly pour the mixture over the bread cubes, gently pressing with a spatula to help the liquid absorb. Let stand for 10 minutes.
Make a bain-marie (water bath) by placing the baking dish in a large roasting pan and filling the pan with enough steaming water to reach halfway up the dish's sides. Cover the roasting pan with foil and cut a few vent holes with a sharp knife.
Bake, covered, for 45 minutes. Remove foil and continue baking 40 - 45 minutes, until bread pudding is lightly browned and custard is set. Cool for about 15 minutes, dust with powdered sugar, and serve with warm bourbon sauce.