Grease a 9x13-inch baking dish with butter or nonstick spray.
Spread the bread cubes evenly in the dish.
In a large mixing bowl, whisk together eggs, milk, sugar, vanilla extract, cinnamon, nutmeg, and salt until well combined.
Pour the mixture evenly over the bread, pressing the bread down lightly to soak up the liquid.
Cover and refrigerate overnight for at least 8 hours.
In the morning, preheat oven to 350°F.
Mix melted butter and brown sugar in a small bowl.
Drizzle brown sugar mixture evenly over the top of the casserole.
Bake uncovered for 45 minutes or until the top is golden brown and the center is fully set.
Remove from oven and let cool slightly before serving.
Serve warm with maple syrup, powdered sugar, or fresh fruit.