Grease a 9x13-inch ceramic baking dish generously with butter. Arrange the cubed brioche bread evenly in the dish, spreading it out into a textured, uniform layer.

In a large mixing bowl, whisk together the eggs, whole milk, heavy cream, maple syrup, vanilla bean extract, ground cinnamon, nutmeg, and sea salt until completely smooth and frothy.

Pour the spiced vanilla custard evenly over the brioche cubes, pressing down gently with a spatula to ensure every piece absorbs the liquid. Cover tightly with foil and refrigerate for at least 4 hours, or ideally overnight (8 to 12 hours).

When ready to bake, preheat your oven to 350°F (175°C). Remove the casserole from the refrigerator to sit at room temperature for 20 minutes.

Prepare the streusel: in a medium bowl, combine flour, brown sugar, cinnamon, pecans, and cold cubed butter. Use a pastry cutter or clean fingers to work the butter in until coarse, pea-sized crumbs form.

Sprinkle the crunchy cinnamon streusel generously over the top of the soaked brioche casserole.

Bake uncovered at 350°F (175°C) for 40 to 45 minutes until the center is set, puffed, and the streusel top is deeply golden brown and bubbling at the edges.

Let rest for 10 minutes before dusting with powdered sugar. Serve warm with pure warm maple syrup and fresh seasonal berries.