Spray a 9x13-inch glass baking dish with nonstick spray.
Layer the bread pieces in the prepared baking dish.
Whisk the eggs, milk, cream, cake mix, custard cinnamon, and vanilla. Pour the mixture over the bread.
Cover and refrigerate overnight or for several hours.
Stir the flour, brown sugar, topping cinnamon, and salt, then mix in the softened butter. Refrigerate the topping with the casserole.
Heat the oven to 350 degrees F.
Uncover the casserole and scatter the topping over it.
Bake for 1 hour, until the center is set and the edges are browned.
Serve with butter, whipped cream, berries, or maple syrup.