Prepare the casserole, by greasing a 9 x 13 inch pan with cooking spray.
Cut the bread into cubes, about 1 inch in size.
Spread half of the cubes into the prepared baking pan and top evenly with half of the blueberries.
Add the remaining bread cubes followed by the remaining blueberries. Set aside.
Whisk the eggs, milk, brown sugar, cinnamon, and vanilla together until very smooth.
Pour evenly over the bread.
Cover the pan very tightly with plastic wrap and place in the refrigerator for at least 3 hours - overnight. Overnight is best.
Preheat oven to 350 degrees. Remove pan from the refrigerator and uncover.
Prepare the topping, by whisking the brown sugar, flour, and cinnamon together in a medium bowl.
Cut in the cubed butter with a pastry blender until crumbly.
Sprinkle the topping evenly over the soaked bread.
Bake for 50-60 minutes or until golden brown on top. Serve immediately.
Cover the cooled leftovers tightly and store in the refrigerator for 2-3 days. Enjoy!