1.Grease a 19cm x 29cm lamington pan.
2.Combine Mars Bars and butter in large saucepan, stir constantly over low heat without boiling until mixture is smooth, stir in Rice Bubbles. Press mixture evenly into prepared pan.
3.To make toping, melt chocolate and butter in pan over hot water, stir until smooth. Spread topping over mixture. Refrigerate until set before cutting.