Butter the bottom and sides of an 8-inch-square baking dish. Remove the peel from the sweet potatoes and cut crosswise into 1/2-inch-thick slices. Arrange the sweet potato and apple slices in the prepared pan, alternating. Pour the whiskey-pecan mixture over the top. Cover with aluminum foil and bake 15 minutes. Remove the foil and baste the top with the whiskey sauce that's collected in the bottom of the pan; continue baking until the apples are soft and glazed, about 18 more minutes.