Prep Baking Dish: Grease a 9x13-inch ceramic or glass baking dish generously with softened butter. Arrange the cubed brioche bread evenly across the dish.

Whisk Pumpkin Custard: In a large bowl, whisk together eggs, whole milk, heavy cream, pumpkin puree, brown sugar, vanilla extract, pumpkin pie spice, and sea salt until completely smooth and emulsified.

Pour & Chill Overnight: Pour the spiced pumpkin custard evenly over the brioche cubes. Press down gently with a spatula so every piece absorbs the liquid. Cover tightly with foil and refrigerate for at least 4 hours, or ideally overnight (up to 24 hours).

Preheat Oven: Remove casserole from refrigerator 20 minutes prior to baking to take the chill off. Preheat oven to 350°F (175°C).

Prepare Pecan Streusel: In a medium bowl, combine flour, brown sugar, cinnamon, and cold cubed butter. Work together with a pastry blender or fingers until coarse crumbs form, then stir in chopped pecans.

Top & Bake: Scatter the pecan streusel evenly over the soaked casserole. Bake uncovered at 350°F for 45 to 55 minutes until golden brown, puffed, and the center is set (toothpick comes out with no wet custard).

Rest & Serve: Let rest for 10 minutes before slicing. Dust with powdered sugar and serve warm with a generous pour of warm pure maple syrup!