Slice Brioche bread into 1-inch cubes, then spread them out, along with raisins (optional), in a greased 9'x13' rectangular baking dish.
In a large mixing bowl, bring together milk, eggs, egg yolks, sugar, and vanilla extract. Whisk to combine.
Pour mixture onto baking dish filled with brioche bread and raisins. Let it sit for 15 minutes.
Pre-heat oven to 350℉.
Bake for 55-60 minutes, or until center is fully cooked. If the top is too browned at any time, cover with aluminum foil.
Remove from oven and let it sit for 5 minutes.
Optional: sprinkle top with powdered sugar. Serve and enjoy!