Grease a 9x13-inch dish with butter or spray; arrange challah evenly.
Whisk eggs, cream, milk, maple syrup, vanilla, cinnamon, nutmeg, and salt smooth in a large bowl.
Pour evenly over bread; press gently with a spatula to moisten all cubes.
Drizzle with butter; sprinkle with optional pecans and brown sugar.
Cover with plastic; refrigerate at least 2 hours or overnight to soak.
Heat oven to 350°F; remove plastic and bake uncovered 45-50 minutes until golden and center set. A knife inserted should come out clean.
Rest 10 minutes; serve warm with extra syrup, berries, or powdered sugar.
To freeze, prepare through step 4, wrap tightly with plastic and foil, and freeze up to 2 months. Thaw refrigerated overnight before baking as directed.