Grease a 9x13 inch baking dish with butter.
Spread the bread cubes evenly in the dish.
In a large bowl, whisk together eggs, milk, heavy cream, sugars, vanilla, cinnamon, and salt.
Pour the mixture over the bread, pressing down slightly to soak.
Cover and refrigerate overnight (or at least 4 hours).
Preheat oven to 350°F (175°C).
Bake for 45–50 minutes until golden brown and cooked through.
Let cool for a few minutes, then serve with maple syrup.