Cook sweet potatoes in boiling water to cover 45 minutes; drain and cool slightly. Peel sweet potatoes, and cut into 1/4-inch-thick slices. Peel, core, and thinly slice apples into rings.
Combine brown sugar and next 3 ingredients.
Arrange half of potato slices and half of apple slices in a lightly greased 13- x 9-inch baking dish. Sprinkle with half of brown sugar mixture, and dot evenly with half of butter. Repeat procedure with remaining ingredients.