Grease a 9×13-inch baking dish with butter or non-stick cooking spray. Cut your bread into uniform 1-inch cubes. Spread the cubes evenly into the prepared baking dish, ensuring they are distributed well to catch all the custard later.

In a large mixing bowl, whisk together the eggs, whole milk, pumpkin puree, brown sugar, vanilla extract, pumpkin pie spice, and salt. Whisk vigorously until the mixture is completely smooth and the pumpkin is fully incorporated into the liquid.

Pour the pumpkin mixture evenly over the bread cubes in the baking dish. Use a spatula to gently press down on the bread, ensuring every piece is submerged or coated in the liquid. Cover the dish tightly with plastic wrap or foil and refrigerate for at least 4 hours, though overnight is highly recommended for the best texture.

In a small bowl, combine the flour, brown sugar, and cinnamon. Use a pastry cutter or your fingers to work the cold, cubed butter into the mixture until it resembles coarse crumbs. If using pecans, toss them in now. Store this topping in a small container or bag in the fridge until morning.

Preheat your oven to 350°F (175°C). Remove the casserole from the refrigerator and uncover it. Sprinkle the streusel topping evenly over the soaked bread. Bake for 45 to 50 minutes, or until the casserole is set in the center and the top is golden brown and slightly crisp.

Let the casserole rest for 5 to 10 minutes after removing it from the oven to let the custard finish setting. Slice into large squares and serve warm. Top with maple syrup, a dollop of whipped cream, or a dusting of powdered sugar for the ultimate fall breakfast experience.