Preheat your oven to 350°F (175°C).
Grease a 9x13 inch baking dish with the unsalted butter, making sure to coat it evenly to prevent sticking.
Slice the day-old bread into thick slices, about 1 inch thick, and arrange them in the greased baking dish. You may overlap the slices if necessary.
In a large mixing bowl, whisk together the eggs, whole milk, heavy cream, granulated sugar, vanilla extract, ground cinnamon, and salt until well combined.
Pour the egg mixture evenly over the bread slices, ensuring all pieces are soaked well. Gently press down on the bread with a spatula to ensure the liquid soaks into each piece.
Cover the baking dish with plastic wrap or aluminum foil and let it sit at room temperature for about 30 minutes, or refrigerate for up to overnight.
After resting, remove the cover and place the baking dish in the preheated oven. Bake for 30 minutes, or until the top is golden brown and the center is set.
Once baked, remove from the oven and let it cool for a few minutes. Slice into squares or serve directly from the dish.
Serve warm with maple syrup, fresh fruit, or a dusting of powdered sugar as desired.