In large bowl, measure cereal; set aside. In 2-gallon resealable food-storage plastic bag, add powdered sugar; set aside.
In small microwavable bowl, microwave 1/2 cup of the chocolate chips and the butter uncovered on High 30 to 45 seconds; stir until smooth. Stir in vanilla. Pour mixture over cereal, stirring until evenly coated. Pour into plastic bag of powdered sugar. Seal bag; shake until well coated. Spread on waxed paper to cool, about 30 minutes.
Add remaining 1/2 cup chocolate chips, the marshmallows and graham cracker pieces. Store in airtight container.