Preheat oven to 350 degrees F. Coat the inside of a 9-inch x 13-inch baking dish with non-stick cooking spray.
Cut the French bread loaf into 1-inch cubes (about 5 to 6 cups). Place the bread cubes into the prepared dish.
In a large bowl, whisk together milk, eggs, 2 tablespoons sugar, salt, and vanilla. Pour the custard mixture over the bread cubes. Gently press down on the bread to help it soak up the custard mixture. Let the casserole rest for 10 minutes.
Break butter into small pieces and place it all over the top of your casserole. Combine 2 tablespoons sugar with cinnamon in a small bowl. Sprinkle evenly over the top of the casserole.
Bake for 40 to 50 minutes, until the top is golden brown and crispy.
Remove from the oven. You can serve it immediately or let the casserole rest for 5 minutes for easier cutting. Top with syrup and some berries, if desired.