Generously butter a 9x13 inch baking dish. Spread the cubed brioche bread evenly across the dish.

In a large bowl, whisk together the eggs, whole milk, heavy cream, light brown sugar, vanilla extract, cinnamon, and salt until completely smooth.

Pour the custard evenly over the brioche cubes, gently pressing the top pieces down with a spatula so every cube is moistened. Cover tightly and refrigerate for at least 2 hours or overnight (up to 12 hours).

When ready to bake, preheat your oven to 350°F (175°C) and let the casserole sit on the counter for 20 minutes while the oven warms up.

In a medium bowl, combine the flour, brown sugar, and cinnamon. Cut in the cold cubed butter with a fork or your fingertips until coarse crumbs form, then stir in the chopped pecans.

Sprinkle the pecan streusel evenly over the soaked brioche. Bake uncovered for 42-48 minutes until puffed, golden brown on top, and set in the center.

Cool for 10 minutes, dust with powdered sugar, and serve warm with pure maple syrup and fresh berries.