Line a 32cm x 23cm cake tin with baking parchment.
Melt the butter in a large saucepan over a medium heat then add the marshmallows and stir constantly until melted. Stir in the Rice Krispies® until well coated then transfer to the prepared tin and press down lightly with the back of a spoon to compact.
Place in the fridge to set for at least 1 hour. Once set, remove from the fridge and lift out of the tin using the baking parchment to help you. Cut into squares and serve.