Melt the butter or margarine in a large
pan over a low heat. Add the marshmallows and cook, stirring
continuously, until the marshmallows have melted and the mixture is
syrupy. Remove from the heat and add the vanilla essence. Stir in the
rice cereal until evenly coated. Press into a 23 cm/9 in square tin
(pan) and cut into bars. Leave to set.