Grease a 9x13-inch baking dish with butter or cooking spray. Arrange the brioche slices in a single layer in the dish, slightly overlapping if necessary.
In a large mixing bowl, whisk together the eggs, whole milk, heavy cream, vanilla extract, granulated sugar, salt, and ground cinnamon until well combined.
Pour the egg mixture evenly over the brioche slices, ensuring all pieces are soaked. Cover the dish with plastic wrap and refrigerate overnight.
In the morning, preheat your oven to 350°F (175°C). Remove the plastic wrap from the baking dish.
Sprinkle the brown sugar evenly over the top of the soaked bread.
Bake in the preheated oven for 30-35 minutes, or until the top is golden brown and the custard is set.
Let it cool for a few minutes before serving. Optionally, serve with fresh berries on the side.