Prepare the Baking Dish:

1. Spray a 9×13-inch baking dish with nonstick spray.
2. Spread half of the bread cubes evenly in the dish.

2. Layer the Ingredients:

1. Scatter cubed cream cheese or mascarpone over the bread.
2. Sprinkle blueberries evenly on top.
3. Add the remaining bread cubes, creating a layered effect.

3. Make the Egg Mixture:

1. In a large mixing bowl, whisk together eggs, milk, vanilla extract, and maple syrup until smooth.
2. Pour the egg mixture evenly over the layered bread and blueberries.
3. Cover with plastic wrap and refrigerate overnight.

4. Bake the Casserole:

1. Preheat the oven to 350°F.
2. Remove the casserole from the fridge 30 minutes before baking.
3. Cover with foil and bake for 30 minutes.
4. Remove foil and bake for an additional 25-30 minutes, or until the center is set and the top is golden brown.

5. Prepare the Blueberry Sauce:

1. In a medium saucepan, combine sugar and cornstarch.
2. Add water and cook over medium heat, stirring constantly until the mixture thickens and boils.
3. Stir in blueberries and simmer for 10 minutes.
4. Add butter and stir until melted.

6. Serve:

1. Slice the casserole into portions and drizzle with warm blueberry sauce.
2. Garnish with powdered sugar or whipped cream, if desired.