1. In a saucepan, melt brown sugar and butter over medium heat until bubbly and smooth. Pour into a greased 9×13-inch baking dish to create a caramel base.

2. Slice the brioche or challah into thick pieces and arrange over the caramel in the baking dish.

3. In a large bowl, whisk together eggs, heavy cream, milk, vanilla extract, salt, and cinnamon (if using).

4. Pour the custard mixture evenly over the bread, pressing slightly to ensure even absorption.

5. Cover the dish and refrigerate overnight, or for at least 8 hours.

6. In the morning, remove from the fridge and let sit at room temperature for 15–20 minutes.

7. Preheat oven to 350°F (175°C).

8. Bake uncovered for 35–45 minutes, until puffed and golden with a set center.

9. Let rest for 10 minutes before serving.