Pour the Rice Chex cereal into a very large mixing bowl and set aside. Line a large baking sheet with parchment paper.
In a microwave-safe bowl, combine the chocolate chips, peanut butter, and butter. Microwave in 30-second intervals, stirring between each, until completely smooth and melted (about 90 seconds total).
Stir the vanilla extract into the melted chocolate mixture until combined.
Pour the chocolate mixture over the cereal and gently fold with a rubber spatula until every piece is evenly coated. Work quickly as the mixture will start to set.
Transfer the coated cereal to a large resealable plastic bag or a very large bowl with a lid. Add 1 cup of powdered sugar, seal, and shake vigorously for 30 seconds.
Add the remaining 1 cup powdered sugar and shake again until all pieces are thoroughly coated in white powder.
Spread the mixture onto the prepared baking sheet in a single layer and let cool completely for 15-20 minutes.
Once fully cooled, gently fold in the chopped Reese's Peanut Butter Cups. Transfer to an airtight container and try not to eat it all at once.
