Slice Brioche loaf into about 3/4 inch thick slices.
Grease a 9×13 baking dish generously with butter all over the bottom and up the sides. (You can also use a deep 9×9 if needed, just add about 5 extra minutes to bake time.)
Stack and layer bread slices evenly in the baking dish and have them overlap each other.
In a mixing bowl, whisk together eggs, milk, sugar, vanilla extract, and cinnamon until smooth.
Pour the mixture slowly all over the top of bread and in between each slice. Make sure that each slice is evenly coated with egg mixture.
Cover the dish with aluminum foil and refrigerate overnight.
In the morning:
Set the oven to 350° and place the baking dish into the oven right away. Let the oven preheat with the dish inside so the dish heats up gradually.
Bake French Toast covered for 25 minutes. Take off the foil and bake it uncovered for 15-20 more minutes.
Sprinkle some powdered sugar on top and serve with some maple syrup. (Try Pumpkin Syrup during the holidays.)