Grease a 9x13 pan very lightly with non-stick cooking spray. Set aside.
Cut bread slices into 1 inch chunks and stagger them in the prepared baking dish as even as possible. You may need more or less bread to fill your pan depending on thickness/size.
In a large bowl or measuring cup, whisk eggs, milk, heavy cream, sugar, cinnamon and vanilla together very well until combined. Pour custard mixture evenly over bread pieces trying to moisten every piece as well as you can. Tightly cover with plastic wrap and store in the refrigerator over night.
To make the crumble topping, whisk together flour, brown sugar, cinnamon and salt in a medium bowl. Cut in butter until a coarse meal consistency is achieved. Either cover the bowl with saran wrap or place the mixture into a resealable bag to store in the refrigerator overnight with the pan of french toast.
In the morning, preheat your oven to 350 degrees. Uncover the french toast and evenly distribute the crumble on top.
Bake for 45 minutes - 1 hour depending on how gooey and custard-like you want the center to be.
Once done, serve warm with maple syrup or even just a sprinkle of confectioner's sugar on top.