Creating the Overnight Blueberry French Toast Casserole is simple if you follow these step-by-step instructions:

1. Prep the Bread: Cut the brioche or challah bread into cubes. Place the cubed bread in a large mixing bowl.
2. Add Blueberries: Gently fold in the fresh blueberries until evenly distributed throughout the bread cubes.
3. Mix Wet Ingredients: In another bowl, whisk together the eggs, milk, vanilla extract, granulated sugar, ground cinnamon, and salt until smooth and well combined.
4. Combine Mixtures: Pour the egg mixture over the bread and blueberries, ensuring the bread is thoroughly soaked. Gently toss to combine, making sure all the bread cubes are coated.
5. Grease the Baking Dish: Lightly grease a 9×13-inch baking dish with butter. Transfer the bread mixture into the dish, spreading it out evenly.
6. Cover and Chill: Cover the dish with plastic wrap and refrigerate for at least 4 hours, or ideally overnight, to allow the flavors to meld and the bread to absorb the mixture.
7. Preheat the Oven: When ready to bake, preheat your oven to 350°F (175°C).
8. Bake the Casserole: Remove the casserole from the refrigerator. Drizzle the melted butter over the top. Bake in the preheated oven for 40-45 minutes, or until the top is golden brown and a knife inserted in the center comes out clean.
9. Let Cool: Once baked, remove the casserole from the oven and let it cool for 5-10 minutes before serving.
10. Dust and Serve: Dust with powdered sugar if desired and serve with warm maple syrup.

By following these steps, you can easily create an Overnight Blueberry French Toast Casserole that will impress everyone!