Prep Baking Dish & Bread: Grease a 9x13-inch ceramic or glass baking dish generously with butter. Cut the day-old brioche into 1-inch cubes. Spread half of the bread cubes evenly across the bottom of the dish.
Mix Cinnamon Swirl: In a small bowl, whisk together the melted butter, packed dark brown sugar, and 1.5 tablespoons ground cinnamon until a glossy, spreadable cinnamon paste forms. Spoon half of this mixture over the first layer of bread cubes.
Add Remaining Bread & Swirl: Add the remaining brioche cubes on top, pressing down gently. Drizzle the rest of the brown sugar cinnamon mixture all over the top layer.
Whisk & Pour Custard: In a large mixing bowl, whisk together the eggs, whole milk, heavy cream, pure vanilla extract, ground cinnamon, nutmeg, and sea salt until completely smooth and frothy. Pour the custard evenly over the entire baking dish, making sure every bread cube is moistened. Gently press cubes down with a spatula. (Optionally cover and chill overnight in the fridge for an effortless make-ahead brunch!)
Bake to Golden Perfection: Preheat oven to 350°F (175°C). Bake uncovered for 35 to 40 minutes, until the top is puffed and deeply golden-crisp on the edges, and the custard is set in the center (no longer liquid). Tent loosely with foil during the last 10 minutes if browning quickly.
Whisk Glaze & Drizzle: While the bake rests for 10 minutes, beat softened cream cheese, butter, powdered sugar, vanilla, and warm milk in a bowl until smooth and pourable. Drizzle generously over the warm French toast bake. Serve warm with fresh berries and maple syrup.