01 - Preheat the oven to 350°F.
02 - Wash and core the apples, creating a cavity without cutting all the way through the bottom.
03 - In a small bowl, combine brown sugar, walnuts (if using), cinnamon, vanilla extract, orange zest, and a pinch of salt.
04 - Place apples in a baking dish. Fill each cavity with the sugar-nut mixture and drizzle with orange juice.
05 - Top each apple with a piece of butter.
06 - Pour 1/4 cup water into the bottom of the baking dish.
07 - Bake for 35 minutes, or until apples are tender but still hold their shape.
08 - Serve warm, spooning some of the pan juices over each apple.