In 1-quart microwavable bowl, microwave chocolate chips, peanut butter and butter uncovered on High 1 minute to 1 minute 30 seconds, stirring every 30 seconds, until smooth. Stir in vanilla until well mixed. Pour chocolate mixture over Chex™ cereal in large bowl; stir until well coated. Pour into 2-gallon resealable food-storage plastic bag; add powdered sugar. Seal bag; shake until well coated.