To make stale brioche bread, either allow the cubes to sit out on the counter all day or place them on a baking sheet and bake at 300 degrees for 15 minutes stirring halfway through.
Grease a 9x13 inch baking dish with butter. Place half of the bread cubes in the dish.
Whisk together the eggs, milk, heavy cream, brown sugar, cinnamon, vanilla extract, and salt.
Pour half of the custard over the bread cubes. Then, add the other half of bread cubes and top with the rest of the custard. You can move the bread around a bit to cover everything, but avoid moving them too much or they'll break apart.
Cover the dish with aluminum foil and place it in the fridge to chill overnight. (See note #1)
In the morning, pull the casserole from the fridge to come to room temperature for 30 minutes.
Then, preheat the oven to 350 degrees F.
Bake covered for 45 minutes. Then, remove the foil and continue baking for another 30 minutes or until the top is golden brown and the center is set. Top with powdered sugar. (See note #2)