Grease a 9x13-inch baking dish. Arrange bread cubes evenly in the dish.
In a large bowl, whisk together eggs, milk, heavy cream, granulated sugar, vanilla, cinnamon, and nutmeg until smooth.
Pour the egg mixture evenly over the bread cubes, pressing down gently to ensure all pieces are soaked. Cover and refrigerate overnight.
In the morning, preheat oven to 350°F. In a small bowl, mix together melted butter and brown sugar.
Drizzle the butter-sugar mixture over the top of the casserole.
Bake uncovered for 45-50 minutes until golden brown and the center is set. Let stand 5 minutes before serving.
Dust with powdered sugar and serve with warm maple syrup.
To freeze: After step 3, cover tightly with plastic wrap and foil. Freeze up to 2 months. Thaw in refrigerator overnight, then proceed with steps 4-7.