Got leftover panettone? Lucky you! The traditional Italian Christmas bread makes an excellent base for a breakfast casserole. All it needs is a soak in spiced custard and a turn in the oven.
Arrange bread in overlapping layers in a 2 1/2-to 3-quart baking dish. In a large bowl, whisk together eggs, milk, vanilla, salt, cinnamon, and nutmeg until thoroughly combined. Pour egg mixture evenly over bread; press with palms to fully submerge. Cover and let stand at room temperature 1 hour.
Preheat oven to 350 degrees. Bake until puffed, golden brown on top, and set, 45 to 50 minutes (if browning too quickly, tent with foil). Let stand 10 minutes, then serve, dusted with confectioners' sugar and drizzled with maple syrup.
Casserole can be assembled up to 1 day in advance and stored, covered, in refrigerator. Remove from refrigerator 30 minutes before baking.