Preheat your oven to 350°F (175°C). Grease an 8x8-inch baking dish with butter.
In a large mixing bowl, whisk together milk, heavy cream, sugar, eggs, vanilla extract, cinnamon, and salt until smooth.
Add the cubed bread to the bowl and gently mix to coat the bread evenly with the custard mixture. Let it sit for 10 minutes to allow the bread to soak up the custard.
Pour the mixture into the prepared baking dish, spreading it evenly. Bake for 40–45 minutes or until the top is golden brown and the custard is set.
Remove from the oven and let cool slightly before serving. Dust with powdered sugar, if desired.