Prep the Brioche & Butter the Dish: Generously butter a 9x13-inch ceramic or glass baking dish. Cut the brioche loaf into uniform 1-inch cubes. (Pro tip: Day-old, slightly stale brioche absorbs the custard without collapsing into mush; if your bread is fresh, bake the cubes on a sheet pan at 300°F/150°C for 10 minutes to lightly dry out).

Caramelize the Cinnamon Apples: Melt 2 tablespoons of butter in a skillet over medium heat. Add the sliced Honeycrisp apples, 2 tablespoons of dark brown sugar, and 1/2 teaspoon cinnamon. Sauté gently for 4 to 5 minutes until the apples release their juices and soften with caramelized golden edges. Remove from heat.

Whisk the Spiced Vanilla Custard: In a large mixing bowl, vigorously whisk together the 6 eggs, whole milk, heavy cream, 1/4 cup maple syrup, vanilla bean extract, 1.5 teaspoons cinnamon, nutmeg, and sea salt until completely smooth, frothy, and emulsified.

Layer & Soak: Scatter half of the brioche cubes across the bottom of the prepared baking dish. Distribute the sautéed caramelized apples evenly over the bread layer, then top with the remaining brioche cubes. Pour the rich vanilla cinnamon custard slowly and evenly over the entire dish, pressing down gently with a spatula so every cube drinks in the custard. (At this stage, you can cover and refrigerate overnight for up to 12 hours!).

Top with Pecan Streusel & Bake: Preheat oven to 350°F (175°C). In a small bowl, stir together the chopped pecans, remaining brown sugar, rolled oats, 1/2 teaspoon cinnamon, and 3 tablespoons melted butter until clumpy. Scatter the streusel evenly over the top of the casserole. Bake uncovered for 32 to 36 minutes, until the center is set, the brioche peaks are golden-brown and puffed, and the edges bubble. Rest for 5 minutes, dust with powdered sugar, and serve with warm maple syrup.