• Preheat oven to 375°F (190°C) and generously grease a 9x13 casserole dish (see note) with butter.
• Evenly layer Challah pieces into a deep 9x13 casserole dish. Set aside.
  1 14oz loaf Challah bread, cut into 1" cubes
• Combine melted butter, maple syrup, vanilla extract, ground cinnamon, and salt in a large bowl and whisk until well-combined.
  4 Tablespoons unsalted butter, ¼ cup maple syrup, 1 Tablespoon vanilla extract, 1 ½ teaspoons ground cinnamon, ¾ teaspoon table salt
• Add sugars and eggs and whisk well.
  ¼ cup sugar, ¼ cup light brown sugar, 7 large eggs
• Add milk and heavy cream and whisk until all ingredients are well-combined.
  2 cups whole milk, ½ cup heavy cream
• 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.

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)
  ⅓ cup all-purpose flour, ½ cup light brown sugar, 2 ¼ teaspoons ground cinnamon
• Scatter butter pieces evenly over flour mixture and pulse until sandy crumbs form (if no food processor -- use a pastry cutter or fork and knife to cut butter into mixture until coarse crumbs form).
  8 Tablespoons salted butter

Assemble & Bake

• Let the casserole sit for 15-20 minutes or longer, until the bread has absorbed most of the custard. (See note if you would like to store the casserole to bake another day).
• 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.