Lightly grease a 9x13 inch baking dish with 1 tablespoon of unsalted butter. Arrange the 8 cups of cubed challah bread evenly in the prepared dish.

In a large bowl, whisk together 8 large eggs, 2 1/2 cups of whole milk, 1/2 cup of granulated sugar, 2 teaspoons of pure vanilla extract, 1 teaspoon of ground cinnamon, and 1/4 teaspoon of kosher salt until well combined and smooth.

Pour the egg mixture evenly over the bread cubes in the baking dish, ensuring all bread is moistened. Sprinkle 2 cups of fresh blueberries over the top.

Cover the baking dish tightly with plastic wrap and refrigerate for at least 4 hours, or preferably overnight, to allow the bread to fully absorb the custard.

About an hour before baking, remove the casserole from the refrigerator to come to room temperature. Preheat your oven to 375°F (190°C).

For the streusel topping, in a small bowl, combine 1/4 cup of all-purpose flour, 1/4 cup of packed light brown sugar, and 1/2 teaspoon of ground cinnamon. Cut in 3 tablespoons of cold unsalted butter until the mixture resembles coarse crumbs. Sprinkle this topping evenly over the casserole.

Bake for 45-50 minutes, or until the casserole is puffed, golden brown, and the center is set (a knife inserted into the center should come out clean). If the top browns too quickly, you may loosely tent it with foil.

Remove from oven and let stand for 5-10 minutes before serving. Serve warm with a dusting of powdered sugar and a drizzle of pure maple syrup, if desired.