In a microwave safe bowl, melt together the white chocolate chips, peanut butter, and unsalted butter. Microwave on 20 second increments, stirring well in between.
Add the pumpkin puree, vanilla extract, and pumpkin pie spice, and stir well.
Add the chex cereal to a large bowl. Pour the mixture over the chex cereal, and stir to coat well.
Add the powdered sugar to two large plastic zip bags.
Divide the coated cereal between the bags, and shake well to coat.
Dump the cereal onto a large baking sheet, and allow it to set for 1 hour.
Add in the Reese's pieces before serving and enjoy.
Notes
Storage: Store in an airtight container at room temperature for up to 1 week.