Preheat your oven to 350°F (175°C). Generously grease a 9x13-inch baking dish with butter or non-stick cooking spray.
Spread the cubed challah bread in an even layer in the prepared baking dish.
In a large mixing bowl, whisk together the whole milk, pumpkin puree, eggs, 1/4 cup brown sugar, maple syrup, vanilla extract, pumpkin pie spice, 1/2 teaspoon cinnamon, and kosher salt until the mixture is completely smooth and well-combined.
Slowly pour the pumpkin custard mixture evenly over the bread cubes in the baking dish. Use a spatula to gently press down on the bread, ensuring all pieces are soaked with the custard.
In a small bowl, combine the melted butter, all-purpose flour, granulated sugar, 1/4 cup brown sugar, chopped pecans, and 1/2 teaspoon cinnamon to create a streusel topping.
Sprinkle this topping evenly over the soaked bread. Bake for 45-55 minutes, or until the casserole is set in the center, the top is golden brown, and the edges are slightly crisp.