Spray 9x13-inch pan with nonstick spray.

Set aside.

Place butter in large bowl.

Microwave on HIGH 30 seconds or until melted.

Add marshmallows; stir to coat well with butter.

Microwave 1 minute; stir.

Continue microwaving 30 to 45 seconds, just until completely melted.

Stir in vanilla.

Immediately add cereal; stir to mix well and completely coat cereal.

Press cereal mixture evenly into prepared pan.

Cool completely. Cut into 24 bars.