Mix chocolate chips, peanut butter and butter in microwave-safe bowl. Microwave on high 1 minute. Stir. Microwave 30 sec longer or until mixture can be stirred smooth. Stir in vanilla.
Pour chocolate mixture over Chex cereal and stir until evenly coated. Pour into 2 gallon food-storage plastic bag. Add powdered sugar and shake until well coated. Spread on wax paper and let cool about 30 minutes. Store in air-tight container.