1.Grease 20cm x 30cm (8-inch x-12 inch) rectangular pan; line base and long sides with baking paper, extending paper 5cm (2 inches) over sides.
2.Combine marshmallows and chocolate in medium saucepan; stir over low heat until smooth.
3.Place rice bubbles in large bowl; working quickly, stir in marshmallow mixture. Press mixture firmly into pan; smooth surface with wet hands.
4.Refrigerate 3 hours before cutting.