Place butter in large microwave-safe bowl. Microwave on HIGH 30 seconds or until melted. Stir in marshmallows. Microwave 1 minute; stir. Continue microwaving, about 30 seconds, until completely melted.

Stir in vanilla. Add cereal; stir to mix well and completely coat cereal.

Drop cereal mixture by 2-tablespoon (1/8-cup) measure onto waxed paper or baking sheet sprayed with nonstick spray. Press to flatten slightly.

Cool completely. Store covered at room temperature up to one week.