Melt butter in large deep pan over low heat.
Add marshmallow and stir until completely melted.
Remove from heat add rice cereal and stir until they are well coated.
Using buttered spatula press mixture into buttered pan 13x9x2.
VARIATION: Stir in 1/4 cup of peanut butter into marshmallow mixture before cereal is added.
You could also use colored marshmallows too.