Generously butter a 9x13-inch ceramic baking dish. Arrange the cubed brioche bread evenly throughout the dish in a single, relaxed layer.

In a large mixing bowl, whisk together the eggs, whole milk, heavy cream, pumpkin purée, maple syrup, brown sugar, pumpkin pie spice, vanilla extract, and salt until completely smooth and velvety.

Slowly pour the spiced custard evenly over all the bread cubes, pressing down gently with a spatula so every piece absorbs the liquid.

Cover the dish tightly with foil or plastic wrap and refrigerate for at least 4 hours, or ideally overnight (up to 12 hours) to allow the custard to fully permeate the brioche.

In a small bowl, combine flour, brown sugar, ground cinnamon, and chopped pecans. Add the cold cubed butter and cut in using a pastry cutter, fork, or clean fingertips until pea-sized crumbles form. Keep chilled until ready to bake.

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

Scatter the pecan streusel evenly across the top of the soaked casserole.

Bake uncovered for 45 to 50 minutes until puffed, golden brown on top, and the custard center is just set without wobbling. (Tent loosely with foil during the final 10 minutes if the pecans brown too quickly).

Let rest for 10 minutes before slicing. Dust with confectioners' sugar and serve warm with plenty of maple syrup.