Grease a 9x13-inch ceramic or glass baking dish generously with softened butter. Spread the 1-inch cubed brioche bread evenly into the prepared dish, creating a rustic textured surface with plenty of bread peaks.

In a large mixing bowl or 4-cup glass pitcher, vigorously whisk the eggs, whole milk, heavy cream, granulated sugar, vanilla bean paste, 1 tsp cinnamon, nutmeg, and salt until smooth and aerated.

In a separate small bowl, stir together the melted butter, dark brown sugar, 1 tbsp cinnamon, and 1 tbsp flour until a thick, glossy spiced ribbon forms.

Pour the egg custard evenly over all the brioche cubes in the baking dish, ensuring every piece of bread is moistened. Use a fork or spatula to gently press down on any dry surface peaks.

Drizzle the warm cinnamon-brown sugar mixture in ribbons all over the top, using a butter knife to gently swirl it into the bread crevices so it mimics an authentic cinnamon roll swirl.

Cover and let soak for at least 30 minutes at room temperature, or cover tightly and refrigerate overnight (up to 12 hours) for the ultimate make-ahead holiday breakfast.

Preheat oven to 350°F (175°C). If chilling overnight, let casserole sit at room temperature for 15 minutes while the oven preheats.

Bake uncovered for 40 to 45 minutes, until the center is set (not jiggly), the top brioche points are puffed and deep golden brown, and the cinnamon sugar is bubbling caramel-like around the edges.

While the casserole bakes, beat the softened cream cheese, butter, powdered sugar, vanilla, and 2 tablespoons milk with an electric hand mixer until silky, smooth, and easily pourable.

Let the warm casserole rest for 5 to 10 minutes out of the oven. Generously drizzle the warm cream cheese glaze over the top, letting it melt into every fluted cinnamon crevice.

Slice into generous squares, garnish with fresh ripe blackberries, and serve warm with a drizzle of pure maple syrup and hot coffee!