Prep the Bread & Baking Dish: Grease a standard 9x13-inch ceramic or glass baking dish generously with softened butter. Arrange the cubed brioche bread evenly throughout the baking dish.
Whisk the Spiced Pumpkin Custard: In a large mixing bowl or pitcher, whisk together the eggs, whole milk, heavy cream, pumpkin puree, maple syrup, brown sugar, vanilla extract, pumpkin pie spice, cinnamon, and salt until completely smooth, velvety, and well incorporated.
Soak the Brioche: Pour the pumpkin custard mixture slowly and evenly over the cubed brioche in the baking dish, ensuring every cube is touched by custard. Gently press down on top with a silicone spatula so the bread absorbs the liquid. (At this stage, you can cover and refrigerate overnight for up to 12 hours, or proceed directly to baking after a 15-minute rest).
Make the Crunchy Pecan Streusel: In a medium bowl, combine the flour, brown sugar, cinnamon, and chopped pecans. Add the cold cubed butter and cut in using a pastry cutter, fork, or your fingertips until coarse, crumbly, pea-sized crumbs form. Scatter the streusel evenly over the soaked brioche.
Bake to Golden Perfection: Preheat your oven to 350°F (175°C). Bake the casserole uncovered for 42 to 48 minutes, until the top is deeply golden brown, puffy, the streusel is crisp, and a knife inserted into the center comes out clean with no liquid custard.
Cool & Serve: Allow the bake to cool and settle for 10 minutes on a wire rack. Dust lightly with powdered sugar, slice into generous squares, and serve warm with a generous drizzle of pure maple syrup and whipped cinnamon butter.