Preheat your oven to 350°F (175°C).
In a large mixing bowl, whisk together the eggs, whole milk, heavy cream, granulated sugar, vanilla extract, cinnamon, nutmeg, and salt until well combined.
Place the bread cubes into the 9x13 inch baking dish. Pour the egg mixture evenly over the bread, ensuring all pieces are well soaked.
Cover the baking dish with aluminum foil and refrigerate for at least 30 minutes or overnight for best results.
Once ready to bake, take the dish out of the refrigerator and let it sit at room temperature for about 10 minutes.
Drizzle the melted butter over the top of the casserole.
Bake, covered with foil, for 30 minutes. Then, remove the foil and bake for an additional 15 minutes, or until the casserole is golden and set in the middle.
Allow to cool for a few minutes before slicing. Optionally, dust with powdered sugar and serve with maple syrup.