Grease a 9x13-inch baking dish with butter. Cut the bread into 1-inch cubes and arrange them evenly in the dish.

In a large mixing bowl, whisk together the eggs, milk, heavy cream, brown sugar, cinnamon, vanilla extract, and salt until smooth.

Pour the custard mixture evenly over the bread cubes, pressing down gently with a spatula to ensure all pieces are coated.

Cover the dish with plastic wrap and refrigerate overnight (or for at least 6 hours).

The next morning, preheat the oven to 350°F (175°C). Remove the casserole from the fridge and let it sit at room temperature while the oven heats up.

Bake uncovered for 45-55 minutes, or until the top is golden brown and the center is set. A knife inserted into the center should come out clean.

Let the casserole cool for 5-10 minutes before serving. Garnish with your favorite toppings, such as powdered sugar, fresh fruit, or maple syrup.