Grease a 9x13-inch ceramic baking casserole dish generously with softened butter. Arrange the cubed brioche bread evenly into the dish, creating a slightly overlapping, fluffy single layer.

In a large mixing bowl or 4-cup glass measuring pitcher, whisk together eggs, whole milk, heavy cream, pure maple syrup, dark brown sugar, vanilla extract, ground cinnamon, nutmeg, and sea salt until completely smooth, frothy, and thoroughly emulsified.

Slowly and evenly pour the spiced egg custard mixture all over the brioche cubes, making sure every piece receives liquid. Gently press down on the bread cubes with the back of a spoon so they drink in the custard. Cover tightly with foil and refrigerate overnight (or at least 4 hours).

In the morning, remove the casserole from the refrigerator and let it sit at room temperature for 25 to 30 minutes while preheating your oven to 350°F (175°C).

Prepare the crunchy streusel topping: in a medium bowl, combine all-purpose flour, dark brown sugar, cinnamon, and chopped pecans. Add the cold cubed butter and work with your fingertips or a pastry cutter until coarse, pea-sized buttery crumbs form. Scatter evenly across the soaked brioche cubes.

Bake uncovered at 350°F (175°C) for 45 to 50 minutes until the casserole is beautifully puffed, the top is deeply golden brown and crunchy, and a knife inserted into the center emerges clean without wet custard.

Let rest for 10 minutes to allow the custard souffle to set. Dust with powdered sugar, scatter with fresh raspberries and blueberries, and serve warm with a generous pour of pure amber maple syrup!