Preheat oven to 375°F. Mist an 8-inch square baking pan with cooking spray. Using a sharp paring knife, cut stem from each apple. Use a spoon or melon baller to scoop out seeds and core, creating a deep cavity but leaving about 1/2 inch of bottom intact. Place apples upright in baking dish.
Combine brown sugar, salt, butter and cinnamon in a small bowl. Use fingers to press ingredients together and work into a thick paste. Mix in walnuts with fingers. Divide mixture and place into cavities of apples.
Drizzle molasses over and around apples. Pour in enough water to just cover bottom of dish. Bake, basting often with pan juices, until apples are tender and filling is golden, 35 to 40 minutes.