Preheat your oven to 350°F (175°C) and lightly grease a 9×13 baking dish with butter or cooking spray.

Slice the brioche into 1-inch cubes and transfer the cubed bread to a large mixing bowl.

In a small mixing bowl, combine the eggs, milk, 3 tablespoons of brown sugar, cinnamon, and salt, and whisk until well combined.

Slowly pour half of the egg mixture (custard) over the brioche. With clean hands, gently fold the brioche over once or twice to coat. Add the remaining custard and fold everything once more with your hands just until the custard has been absorbed and the brioche is well coated.

Gently transfer the brioche to your prepared baking dish and spread it out into an even layer. Sprinkle the remaining brown sugar on top of the casserole and bake until golden brown and the custard has set, about 25-30 minutes.

Remove from the oven, top with desired toppings, and serve immediately.