Spray a 9" x 13" baking dish with cooking spray and line the pan with a single layer of bread slices. Note: you may have to cut a slice in half to get most of the pan covered.
Whisk together eggs, milk, vanilla, cinnamon and salt in a large liquid measuring cup. Pour 1/3 of the total mixture on top of the first layer of bread. Top egg-soaked bread layer with half of the berries.
Place the remaining bread slices on top then pour over the remaining egg mixture, making sure each slice is fully soaked. Sprinkle on remaining berries. Cover pan and refrigerate overnight (or at least 2 hours.)
When you're ready to bake, pre-heat oven to 350 degrees. Remove cover from pan and bake for 30 minutes. Serve with maple syrup and butter if you wish.