Grease a rectangular 18cm+28cm slice pan .Line the base and sides with baking paper .

Combine butter,marshmellows and peanut butter in a large heatproof bowl . sit the bowl over a pan of simmering water,until smooth . Remove . stir in the rice bubbles ( crisps ) . Press mixture over base of the prepared pan .

To make the topping ,combine both ingredients in a small heatproof bowl . Place bowl over a saucepan of simmering water . Stir until smooth .

Spread the topping evenly over the base mix . Refrigerate until set . Slice into squares to serve .