In a microwave-safe bowl, combine the chocolate chips, butter, and peanut butter. Microwave in 30-second bursts, stirring between bursts, until the mixture is smooth. Mix in the optional vanilla extract.

Place the Rice Chex cereal in a large bowl. Gradually pour the melted mixture over the cereal while stirring gently until evenly coated. Allow the mixture to cool to near room temperature.

Add the powdered sugar to the cooled cereal and gently toss until all pieces are lightly covered. Chill the mixture in the refrigerator for 15 minutes, then transfer to an airtight container if not serving immediately.