Preheat oven to 375°F (190°C) and generously grease a 9x13 casserole dish with butter.
Evenly layer Challah pieces into a deep 9x13 casserole dish. Set aside.
Combine melted butter, maple syrup, vanilla extract, ground cinnamon, and salt in a large bowl and whisk until well-combined.
Add sugars and whisk well.
Add eggs and whisk again until ingredients are very well combined.
Add milk and heavy cream and whisk again until all ingredients are well-combined.
Pour mixture evenly over Challah pieces in casserole dish and use a spatula to stir gently so all pieces of bread are able to absorb the custard. Set aside while you prepare your topping.
Combine flour, brown sugar, and ground cinnamon in the canister of a food processor and pulse briefly to combine (if you don't have a food processor, just stir together in a medium-sized bowl).
Scatter butter pieces evenly over flour mixture and pulse until coarse crumbs form (if no food processor -- use a pastry cutter or fork and knife to cut butter into mixture until coarse crumbs form).
IF YOU WANT TO SAVE AND SERVE THE NEXT MORNING: Cover your casserole dish tightly with plastic wrap or foil and place your crumble topping in an airtight container. Place both in the refrigerator.
The next morning, remove your casserole dish and topping from the refrigerator. Preheat oven to 375°F (190°C), uncover casserole dish, sprinkle topping evenly overtop (you may need to give the container a good shake if the topping has settled and started to stick together), and let dish sit while the oven fully preheats. Once preheated, bake*** for 50 minutes, allow to cool for several minutes before serving. Serve topped with maple syrup, if desired.
IF YOU WISH TO SERVE THE FRENCH TOAST CASSEROLE RIGHT AWAY: Let the casserole sit for 15-20 minutes or until the bread has absorbed most of the custard. Sprinkle evenly with topping then transfer to 375°F (190°C) preheated oven and bake*** for 50 minutes. Allow to cool several minutes before digging in. Serve topped with maple syrup, if desired.