In a small bowl toss together cereal and melted butter. Sprinkle over apple mixture. Bake, uncovered, in a preheated 375-degree oven for 35 to 40 minutes, or until apples are tender. To prevent overbrowning, cover top loosely with foil for the last 10 to 15 minutes of baking, if necessary. Serve warm. Serve with yogurt if desired.