Slice Brioche loaf into about 3/4 inch thick slices. (You will get about 12 slices.)
Grease a 9×13 baking dish generously with butter all over the bottom and up the sides.
In a mixing bowl, whisk together eggs, milk, sugar, vanilla extract, and cinnamon until smooth.
Dip each slice of bread into the egg mixture and let it soak for a few seconds. Place it in the baking dish.
Repeat with all the bread slices. Stack and layer bread slices evenly in the baking dish and have them overlap each other.
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:
Place the baking dish into the cold oven and set the oven to 350°F. 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 or other toppings you prefer.
Notes
How To Tell If It’s Done: My first test is always to touch in the middle of and see how jiggly or soft it is. If the middle hasn’t baked through yet, it will be feel squishy. If it starts to feel firm and yields when you gently press into it, it is fully cooked. You can also check the internal temperature using an instant digital thermometer in the center of the bread pudding and aim for 170°F-175°F.