CASSEROLE: Generously grease a 9 x13-inch casserole dish with the butter. Arrange the bread cubes evenly over the bottom of the dish.
In a large bowl, whisk together the eggs, sugar, pumpkin pie spice, cinnamon, salt, and vanilla. Whisk in the milk, cream, and pumpkin puree until well combined. Slowly pour the egg mixture evenly over the bread cubes, making sure that all cubes are well coated. Cover the casserole dish with plastic wrap and refrigerate overnight.
BROWN SUGAR CRUMBLE: In a medium bowl, whisk together the brown sugar, flour, pumpkin spice, cinnamon and salt. Cut in cold butter with a fork or pastry cutter until the mixture is nice and crumbly. Stir in the chopped pecans. Cover and refrigerate until morning
The next morning, preheat oven to 350 degrees. Remove the casserole from the refrigerator and sprinkle with the crumb topping.
Bake uncovered for 45-50 minutes, until puffed and golden all over. Remove from the oven and let cool for 10 minutes, then cut into squares.
Serve with real maple syrup, a dollop of sweetened whipped cream, and a sprinkle of pumpkin pie spice.