1. In a large microwave-safe bowl, combine chocolate chips, peanut butter, and butter. Microwave on high for 1 minute, then stir until smooth. If needed, microwave for an additional 30 seconds.
2. Stir in vanilla extract.
3. Pour the mixture over the Rice Chex cereal in a large bowl. Stir gently until the cereal is evenly coated.
4. Transfer the coated cereal into a large resealable plastic bag. Add powdered sugar, seal the bag, and shake until the cereal is well coated.
5. Spread the cereal out on a baking sheet to cool. Store in an airtight container.