Preheat your oven to 350°F (180°C) and grease a 13x9-inch baking dish with butter or cooking spray.

In a large mixing bowl, whisk together 0.5 cup of sugar, 2 tablespoons of vanilla extract, 0.5 teaspoon of ground cinnamon, 0.5 teaspoon of ground nutmeg, and 6 large eggs until well combined.

Gradually pour in 6 cups of room-temperature eggnog, continuing to whisk until the mixture is smooth and free of lumps.

Cut stale challah or brioche bread into bite-sized cubes and place them evenly in the greased baking dish.

Slowly pour the custard mixture over the bread cubes, making sure to cover them generously.

Gently toss the bread with the custard to ensure every piece is soaked.

Press the bread mixture down gently, cover with aluminum foil, and let it sit for about 30 minutes at room temperature.

Remove the foil and place the baking dish in the preheated oven; bake for 45 to 55 minutes or until puffed and golden brown.

Let it cool for 20-30 minutes before serving.