Grease a 9x13 inch baking dish generously with butter. Arrange the 1-inch bread cubes evenly in the dish. Distribute the 2 cups of blueberries over the top of the bread cubes.

Prepare the Custard: In a large bowl, whisk together the 8 large eggs until frothy. Add the milk, granulated sugar, vanilla extract, cinnamon, and salt. Whisk vigorously until the sugar is fully dissolved.

Assemble and Soak: Carefully pour the custard mixture evenly over the bread and blueberry mixture, ensuring all bread pieces are moistened. Press down gently with a spatula. Cover the dish tightly with plastic wrap or foil. Refrigerate for a minimum of 4 hours, or ideally overnight (8 to 12 hours).

Prepare the Streusel: Whisk together the flour, brown sugar, and 1/2 tsp cinnamon. Cut in the cold butter cubes using a pastry blender or your fingers until the mixture resembles coarse crumbs. Keep refrigerated until ready to use.

Preheat and Top: Preheat your oven to 375°F (190°C). Remove the casserole from the refrigerator 30 minutes before baking. Remove the plastic wrap and sprinkle the prepared streusel evenly over the surface of the casserole.

Bake: Cover the dish loosely with aluminum foil. Bake for 30 minutes covered. Remove the foil and continue baking for an additional 20 to 25 minutes, or until the casserole is golden brown, set in the center, and a knife inserted comes out clean.

Rest: Allow the casserole to rest for 10-15 minutes after removing it from the oven before slicing.

Glaze (Optional): While resting, whisk together the powdered sugar, milk (or cream), and 1/2 tsp vanilla until smooth. Drizzle the glaze generously over the warm casserole just before serving. Serve warm with maple syrup if desired.