Prepare popcorn according to package directions. Remove all unpopped kernels. Place popped corn in large bowl sprayed with cooking spray.
Place marshmallows, butter, sugar and vanilla in microwave-safe bowl. Microwave on HIGH 1 minute. Stir with wooden spoon sprayed with cooking spray. Microwave 1 minute more; stir well.
Pour marshmallow mixture over popcorn. Mix well with wooden spoon sprayed with cooking spray. Moisten hands with water; divide popcorn mixture into 15 equal portions. Form into balls around each chocolate piece, completely enclosing the chocolate. Wrap each popcorn ball in colored cellophane; tie with a ribbon, if desired. Store in tightly sealed container up to 2 days.