Preheat the oven to 350°F (175°C). Grease the baking dish with butter or cooking spray.
Cut the stale bread into 1-inch cubes and place them evenly in the greased baking dish.
In a large mixing bowl, whisk together the eggs, whole milk, heavy cream, sugar, vanilla extract, ground cinnamon, and salt until well combined.
Pour the egg mixture over the bread cubes, ensuring all pieces are coated. Press down gently with a spatula to submerge any bread that floats.
Cover the baking dish with plastic wrap and refrigerate for at least 30 minutes or overnight for best results.
Once ready to bake, remove the plastic wrap and bake in the preheated oven for 45-50 minutes or until the top is golden brown and the center is set.
Let the baked French toast cool for a few minutes before cutting it into squares.
Serve warm with maple syrup and a dusting of powdered sugar, if desired.