Melt butter in large saucepan over low heat. Add marshmallows and stir until completely melted.

Cook over low heat 3 min longer, stirring constantly.

Remove from heat and add rice krispies. Stir until well coated.

Using a buttered spatula, press mixture into a buttered 13 x 9 pan.

Cut into squares when cool.

May stir in 1/4 c peanut butter into marshmallow mixture just before adding cereal