Slice and cut the bread into 1-inch cubes. Let the cubed bread sit out uncovered for a few hours or up to 1 day.
Grease a 9×13-inch baking dish (or any 3.5-4 quart baking dish) or spray with nonstick spray. Spread cubes of bread in the dish.
Whisk pumpkin, brown sugar, pumpkin pie spice, eggs, milk, and vanilla extract together in a large bowl. Pour evenly over bread.
Cover the pan tightly and place in the refrigerator for at least 3 hours and up to 1 day.
Mix the brown sugar, flour, and cinnamon together. Add the cold butter and using a pastry cutter or fork, cut butter into the brown sugar mixture until pea-size crumbles form. Stir in the pecans. Cover and refrigerate until ready to use.
Remove casserole from the refrigerator and preheat oven to 350°F (177°C).
Sprinkle crumb topping evenly over casserole and bake uncovered for 20 minutes, then cover with aluminum foil and bake for an additional 25-35 minutes or until center appears set and is no longer runny.
Remove from the oven and cool for 5 minutes before serving. Drizzle maple syrup on top of the casserole or on individual servings.
Cover leftovers tightly and store in the refrigerator for 2-3 days.
