Prep the Baking Dish & Brioche: Generously grease a 9x13-inch ceramic baking dish with butter. Cut the brioche loaf into bite-sized 1-inch cubes. If the bread is fresh, spread the cubes on a rimmed baking sheet and dry them out in a 300°F (150°C) oven for 8–10 minutes until slightly crisp but not browned. Transfer the dried brioche cubes into the greased baking dish.

Whisk the Spiced Custard: In a large glass measuring bowl or pitcher, vigorously whisk the 6 eggs until smooth. Whisk in whole milk, heavy cream, maple syrup, vanilla bean paste, 1 1/2 teaspoons cinnamon, nutmeg, and sea salt until fully blended and fragrant.

Mix the Cinnamon Sugar Swirl: In a separate small heatproof bowl, combine the melted butter, dark brown sugar, 1 1/2 tablespoons ground cinnamon, and 1 tablespoon flour. Stir with a fork into a smooth, glossy spiced paste.

Assemble & Soak: Pour the creamy egg custard evenly across all brioche cubes, pressing down gently with a spatula so every piece absorbs the liquid. Drizzle the cinnamon roll swirl generously across the entire casserole, using a butter knife to swirl it into the bread crevices. Let stand at room temperature for 15 minutes (or cover tightly and refrigerate overnight).

Bake to Golden Perfection: Preheat oven to 350°F (175°C). Cover the dish loosely with foil and bake for 25 minutes. Remove the foil and bake uncovered for an additional 15 to 18 minutes, until the custard is set, the top is deeply caramelized golden brown, and edges are delightfully crisp.

Whip the Glaze & Drizzle: While baking, beat the softened cream cheese, butter, powdered sugar, vanilla, and warm milk with a hand whisk until silky smooth and pourable. Drizzle warm glaze generously over the casserole straight out of the oven. Serve warm with fresh berries!