Grease a 9x13-inch baking dish and add the cubed brioche bread.
In a large bowl, whisk together eggs, milk, cream, granulated sugar, brown sugar, vanilla, cinnamon, nutmeg, and salt.
Pour the mixture evenly over the bread cubes in the dish. Gently press the bread to soak in the custard.
Cover and refrigerate overnight, or let it sit for at least 30 minutes.
Preheat oven to 350°F (175°C). Bake uncovered for 45–50 minutes or until golden brown and set.
Let it cool slightly before dusting with powdered sugar and serving with maple syrup.