How to Make It
Lightly coat a 13x9-inch baking dish with cooking spray. Line baking dish with foil, leaving a 6-inch overhang on each side: lightly coat foil with cooking spray. Cut the challah crosswise into 1-inch slices, leaving about ½-inch intact at the bottom. Slice lengthwise down the loaf, leaving ½-inch intact at bottom and transfer to the foil-lined baking dish.
In a large liquid measuring cup, whisk the eggs, cream, milk, vanilla, cinnamon, salt, and 2 tablespoons maple syrup until combined. Pry open the slices and carefully pour the batter in between so that the mixture seeps down and absorbs. Wrap the foil tightly around the loaf and refrigerate at least one hour and up to 24 hours.
Preheat oven to 375°F. Unwrap loaf, remove and discard foil and any extra batter, and return loaf to baking dish. Cover dish with a new piece of foil. Bake until the slices are damp but no longer soaked and the center is still soft, about 40 minutes. Meanwhile, combine the melted butter and remaining ⅓ cup maple syrup in a liquid measuring cup or small bowl. Remove the French toast from the oven, remove foil, and pour the butter mixture over the top and in between the slices. Return to oven and bake until edges are crisp and a little darker in color, 20 to 25 minutes. Immediately transfer to a serving platter (otherwise it could stick). Dust with confectioners’ sugar and serve with more maple syrup.