In microwave-safe bowl heat butter and marshmallows on HIGH for 3 minutes, stirring after 2 minutes. Stir until smooth.

Add Rice Krispies cereal. Stir until well coated.

Using buttered spatula or wax paper evenly press mixture into 13 x 9 x 2-inch pan coated with cooking spray. Cool. Cut into 2-inch squares. Best if served the same day.