How to Make It
Coat a 13- x 9-inch baking dish with cooking spray; place bread slices, slightly overlapping, in prepared baking dish. Whisk together eggs, eggnog, milk, granulated sugar, brown sugar, salt, 1 teaspoon of the vanilla, 1 teaspoon of the nutmeg, and 1/2 teaspoon of the cinnamon in a bowl. Pour mixture over bread to fully coat. Cover and refrigerate 8 hours or up to overnight.
Preheat oven to 350°F. Uncover bread mixture. Stir together turbinado sugar and remaining 1/2 teaspoon each nutmeg and cinnamon in a small bowl. Sprinkle over bread mixture. Bake in preheated oven until golden brown, 45 to 50 minutes. Let stand at room temperature 10 minutes.
Meanwhile, cook maple syrup, bourbon, and remaining 1/4 teaspoon vanilla in a small saucepan over low, stirring often, until warmed through, about 2 minutes.
Dust casserole with powdered sugar; serve with maple syrup-bourbon sauce.