To make the panettone stale, cut it into small chunks. Spread them out onto a large shallow baking sheet and let them harden overnight. If you forget to leave them out overnight, don’t worry. Toast the chunks in the oven at 350 degrees F just until hardened, 10 to 15 minutes. Do not let them burn. Allow them to cool before soaking them. Mixing the candied fruits with flour will prevent them from sinking to the bottom of the pan. For a richer bread pudding, soak the chopped candied fruits in rum for 48 hours before draining them, coating in flour and adding to the batter. Because bread pudding is technically a custard, it is preferable to bake it in a bain-marie, which allows it to cook slowly and more evenly. For the bain-marie, place the ring pan in a larger, rectangular baking pan and fill with boiling water to come halfway up the sides of the ring mold pan. Bake until set and slightly jiggly, 45 to 55 minutes.