Prepare bread and pan. Lightly grease a 9 x 13-inch metal baking pan. Cut and weigh the day-old bread, placing the cubes evenly in the pan. If using freshly baked bread, dry the cubes on a tray in a 300°F (150°C) oven for about 10–15 minutes, then cool; this optional preparation adds time beyond the main estimate.

Whisk the custard. Whisk the eggs thoroughly in a large bowl, then whisk in milk, maple syrup, cooled melted butter, vanilla, cinnamon, and salt. Some cinnamon specks will float, but the egg whites should be incorporated. Whisk again just before pouring so the flavorings are not left at the bottom of the bowl.

Soak the bread. Pour the custard evenly over the bread. Use a broad spatula to press the cubes gently into the liquid, coating the exposed edges without crushing the bread into paste. Cover and refrigerate for 30 minutes. For the overnight option, refrigerate 8–12 hours and bake within 24 hours of assembling; keep the raw egg mixture cold.

Begin covered baking. Heat a conventional oven to 350°F (175°C). Keep the metal pan loosely covered with foil and bake for 30 minutes. An overnight-cold casserole can take longer overall than the short-soaked version. Use a pan approved for refrigerator-to-oven use, and do not substitute a cold glass dish for the metal pan.

Uncover and finish. Remove the foil carefully and bake about 15–25 minutes more, until the top is golden and the center no longer holds loose liquid. Check that the center custard reaches at least 160°F (71°C). If the top colors before the center sets, cover loosely again and continue; do not remove an undercooked pan solely because the bread looks brown.

Rest and serve. Rest for 10 minutes, then divide into eight portions with a broad spatula. The casserole settles a little as steam escapes. Serve extra maple syrup separately if desired. Divide leftovers into shallow containers and refrigerate promptly rather than leaving a deep warm pan on the table throughout the morning.