Remove a thin slice from the bottom of each apple so it will stand. Remove a ¾-inch slice from the top. Scoop out the center core of each apple with a melon baller or grapefruit spoon. (Do not cut all the way through the bottom; leave a thick shell on the sides.)
Stir together cranberries (or currants) and brown sugar in a small bowl. Spoon ¼ of the mixture into each apple cavity. Set the apples in a small baking dish. Dot the top of each apple with ½ teaspoon butter. Pour cider (or juice) around the apples and cover the dish tightly with foil.
Bake until the apples are almost tender, 30 minutes. Uncover the pan and baste the apples with the pan juices. Bake uncovered until the apples are tender and the juices are slightly reduced, about 10 minutes longer, basting once or twice more. Spoon the juices into the centers of the apples and serve warm.