Place brioche cubes in a greased 9x13" baking dish.
In a medium bowl, whisk together the eggs, milk, cream, brown sugar, vanilla extract, cinnamon, and salt.
Pour the egg mixture evenly over the bread.
In a small bowl, combine the brown sugar, flour, and cinnamon and then add the cold butter. Cut the butter in with a fork until the. mixture resembles coarse crumbs.
Sprinkle the topping over the French toast casserole.
Cover and refrigerate for at least 1 hour, or preferably overnight.
Preheat the oven to 350 degrees F.
Bake the casserole covered with foil for 30 minutes. Remove the foil and bake another 20-25 minutes, or until the middle is set.
Serve topped with maple syrup or powdered sugar. Enjoy!