Spray an 8 X 8-inch baking dish with cooking spray. Place the bread cubes and halved blackberries into a large bowl and lightly toss. Pour into the prepared baking dish.
In a medium bowl, whisk together eggs, milk, cream, sugar, and vanilla.
Evenly pour mixture over the bread and blackberries. Cover with plastic wrap and place into the refrigerator to sit overnight.
In a medium bowl, combine flour, brown sugar, and cinnamon. Mix thoroughly. Cut butter into small pieces and with a pastry cutter or two forks, blend butter into mixture until it resembles coarse meal.
Place mixture into a plastic bag. Seal and refrigerate overnight. This step can also be done in the morning before placing baking dish into the oven.
Next morning…Preheat oven to 350 degrees F.
Uncover the baking dish and sprinkle the butter mixture evenly around the top. Bake uncovered for 45-50 minutes. 45 minutes will allow for a softer result, 50 a firmer one.
Serve with butter and additional maple syrup, if desired.