Begin by cutting the loaf of challah or brioche bread into 1-inch thick slices. Then arrange the slices in a greased 9x13 inch baking dish, overlapping them slightly.

In a large mixing bowl, whisk together the eggs, whole milk, heavy cream, granulated sugar, vanilla extract, ground cinnamon, and salt until well combined and smooth.

Pour the egg mixture evenly over the bread slices, ensuring that all the pieces are soaked. Press down gently to submerge any floating slices.

Cover the baking dish with plastic wrap and refrigerate overnight (or for at least 4 hours) to allow the bread to absorb the custard mixture.

Preheat your oven to 350°F (175°C) the next morning. Remove the plastic wrap from the baking dish.

Bake the French toast uncovered for 45 minutes, or until it is puffed and golden brown.

Once cooked, remove the baking dish from the oven and sprinkle the brown sugar evenly over the top.

Using a kitchen torch, carefully caramelize the brown sugar until it forms a crispy crust. (If you don't have a torch, you can place the dish under the broiler for 1-2 minutes, watching closely to prevent burning.)

Allow the French toast to cool for a few minutes before serving.