Spray a 9x13-inch baking dish with nonstick spray and set aside.
Cut the bread into 1-inch pieces. Place half into the prepared baking dish. Cut the cream cheese (or mascarpone) into cubes (or scoop with a small cookie scoop) and place on top of the bread. Top with the blueberries and then the remaining bread cubes.
Whisk the eggs in a large bowl. Whisk in the milk, vanilla and syrup until well combined. Pour over the bread cubes. Cover with plastic wrap and refrigerate overnight (I like to place a book on top of the plastic wrap to sink the bread into the liquid). The next morning remove the pan from the fridge 1/2 hour before baking. Preheat oven to 350 degrees F. Remove the plastic wrap, then cover with foil and bake for 30 minutes. Remove the foil and bake another 25 to 30 minutes, or until the center is set and color is golden.
PREPARE THE BLUEBERRY SAUCE:
In a medium saucepan whisk together the sugar and cornstarch. Add the water and bring to a boil over medium heat. Stir constantly and boil for about 3 minutes. Stir in the blueberries and reduce heat to low. Simmer for about 10 minutes or until the blueberries begin to pop. Stir in the butter until melted. Pour warm sauce over individual servings.
Notes
I like to use whole milk- it just makes the dish a little bit more decadent all around.
Mix in chopped fresh, frozen or canned peaches for a peach and blueberry overnight french toast!