In large saucepan, melt butter over low heat; add marshmallows and stir until completely melted. Remove from heat.

Add cereal; stir until well coated.

Using buttered spatula or wax paper, evenly press mixture into 13"x9"x2" pan coated with cooking spray.

Cool; cut into 2 inch squares.

For microwave: heat butter and marshmallows on high for 3 min., stirring after 2 min. Stir until smooth.