Preheat the oven to 350°F (175°C).

Grease a 9x13 inch baking dish with butter or cooking spray.

In a large mixing bowl, combine the cubed stale bread and raisins (if using).

In another bowl, whisk together the milk, heavy cream, eggs, granulated sugar, vanilla extract, ground cinnamon, and salt until smooth.

Pour the custard mixture over the bread cubes, pressing down gently to help absorb the liquid.

Let the mixture sit for 10 minutes to allow the bread to soak up the custard.

Bake for 45 minutes, or until the top is golden brown and a toothpick inserted in the center comes out clean.

Remove from the oven and let cool slightly before serving.