Prep the Bread: Preheat oven to 300°F (150°C). Spread cubed brioche across a large baking sheet and lightly bake for 10 minutes to dry out slightly. Let cool 5 minutes. Grease a 9x13-inch ceramic baking dish with butter and fill with the dried bread cubes.

Whisk the Pumpkin Custard: In a large mixing bowl, whisk together eggs, pumpkin puree, whole milk, heavy cream, maple syrup, vanilla extract, cinnamon, pumpkin pie spice, and salt until completely smooth and emulsified.

Pour & Soak: Slowly pour the pumpkin custard evenly over the cubed bread in the prepared dish. Use a spatula to gently press the top bread cubes down so every piece absorbs the rich liquid. Let rest on the counter for 20 minutes (or cover tightly and refrigerate overnight for up to 14 hours).

Mix the Pecan Streusel: In a small bowl, combine chopped pecans, brown sugar, flour, and cinnamon. Cut in cold cubed butter using a pastry cutter or your fingertips until crumbly and pea-sized clusters form.

Bake to Golden Perfection: Increase oven temperature to 350°F (175°C). Uncover the casserole and scatter the pecan streusel evenly over the top. Bake for 40 to 45 minutes until the center is set, puffed, and the streusel topping is deeply golden and fragrant.

Rest & Serve: Allow the casserole to rest for 10 minutes before slicing into generous squares. Serve warm drizzled generously with pure maple syrup and a cloud of fresh whipped cream!