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 eggs and whisk well.
Add milk and heavy cream and whisk 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.
Scatter butter pieces evenly over flour mixture and pulse until sandy crumbs form.
Let the casserole sit for 15-20 minutes or longer, until the bread has absorbed most of the custard.
Sprinkle casserole evenly with topping then place on a foil-lined baking sheet and transfer to center rack of 375°F (190°C) preheated oven.
Bake for 50 minutes, until browned and mostly set, but center is still slightly jiggly when jostled.
Serve warm, topped with additional maple syrup or whipped cream, if desired.
