Grease a 9- by 13-inch baking dish (or two 8- by 8-inch baking dishes) with butter or coat with nonstick cooking spray.
In a medium saucepan, melt butter over medium heat. Stir in brown sugar and maple syrup; continue stirring as mixture comes to a boil. Remove from heat and pour into prepared baking dish. Arrange the bread cubes over the brown sugar mixture. Sprinkle bread cubes with cinnamon and nutmeg, to taste.
In a large bowl, whisk the eggs and mix in the eggnog and vanilla. Evenly pour this custard mixture over the bread cubes. Press down lightly on the bread to make sure it's all coated/soaked. Tightly cover baking dish with aluminum foil and refrigerate overnight, at least 8 hours.
Remove the baking dish from the refrigerator. Preheat oven to 325°F. Bake, covered, for 40 minutes. Increase oven temperature to 375°F and remove foil. Bake, uncovered, for an additional 5 to 10 minutes or until knife inserted in center of casserole is clean (and mostly dry) and top is puffy and golden brown.
The bread I typically use for this recipe is the in-house baked French Bread from HEB (a Texas grocery store). It weighs about 16 ounces.