Grease a 9x13-inch baking dish with butter or cooking spray. Arrange challah cubes evenly in the dish.
In a large bowl, whisk together eggs, milk, heavy cream, maple syrup, vanilla extract, cinnamon, nutmeg, and salt until smooth and well combined.
Pour the egg mixture evenly over the challah cubes, making sure all bread is moistened. Press down gently with a spatula to help the bread absorb the liquid.
Drizzle melted butter over the top, then sprinkle with pecans (if using) and brown sugar.
Cover with plastic wrap and refrigerate for at least 2 hours or overnight for best results (this step helps the bread soak up all that delicious custard).
When ready to bake, preheat oven to 350°F. Remove plastic wrap and bake uncovered for 45-50 minutes, until golden brown and set in the center. A knife inserted should come out clean.
Let stand for 10 minutes before serving. Serve warm with additional maple syrup, fresh berries, or powdered sugar.
To freeze: Prepare through step 4, then wrap tightly with plastic wrap and foil. Freeze for up to 2 months. Thaw overnight in the refrigerator before baking as directed.