Spray a 9x13" glass baking dish with non-stick spray.
Layer pieces of bread in baking dish.
Whisk together eggs, milk, heaving whipping cream, cake mix, cinnamon and vanilla extract in a large bowl. Pour mixture over bread.
Cover baking dish and store in the fridge overnight or for several hours.
In a small bowl add flour, brown sugar, cinnamon and salt, stir until combined. Mix in I Can't Believe It's Not Butter! to mixture. Store in fridge overnight and several hours.
Preheat oven to 350 degrees.
Remove french toast casserole from fridge, uncover and drop topping by the spoonful randomly across french toast casserole.
Bake for 1 hour until the center is set and edges are browned.
Serve with butter, whipped cream, berries or maple syrup