Start by cutting the bread into 1-inch cubes. Place the bread cubes in the 9x13 inch baking dish, spreading them out evenly.
In a large mixing bowl, whisk together the eggs, milk, heavy cream, sugar, vanilla extract, cinnamon, and salt until well combined.
Pour the egg mixture over the bread cubes, ensuring all the pieces are soaked. Gently press the bread down to help it absorb the liquid.
If using, sprinkle the chopped pecans or walnuts over the top. Cover the dish tightly with aluminum foil and refrigerate overnight (or for at least 4 hours).
Preheat your oven to 350°F (175°C) the next morning.
Remove the foil from the baking dish and drizzle the maple syrup evenly over the top of the casserole.
Bake uncovered for 40-45 minutes, or until the custard is set and the top is golden brown.
Let the casserole cool for a few minutes before cutting into squares. Serve warm, dusted with powdered sugar if desired.