Grease a 9x13-inch ceramic or glass baking dish with 1 tablespoon softened butter. Spread the cubed brioche bread evenly across the prepared dish.

In a large mixing bowl or pitcher, vigorously whisk together the eggs, whole milk, heavy cream, pumpkin puree, brown sugar, vanilla extract, cinnamon, nutmeg, ginger, and kosher salt until completely smooth and velvety.

Pour the spiced pumpkin custard evenly over the brioche cubes. Press the bread 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 12 hours).

Prepare the Pecan Streusel: In a medium bowl, whisk together the flour, brown sugar, and cinnamon. Cut in the cold cubed butter using a pastry blender or your fingers until pea-sized crumbs form. Stir in the chopped pecans.

Preheat oven to 350°F (175°C). Remove the casserole from the refrigerator 20 minutes before baking to take the chill off. Uncover and sprinkle the pecan streusel evenly over the top.

Bake uncovered for 45 to 50 minutes until the top is deep golden-brown, the streusel is crisp, and a toothpick inserted in the center comes out clean and set (custard puffed). Cool for 10 minutes, dust with powdered sugar, and serve warm with pure maple syrup.