Mise en place. Pour cereal into a very large bowl. Put half of powdered sugar and 1/2 tsp. of pumpkin pie spice into a gallon Ziploc bag. Seal and toss to mix. Set aside.
In a medium bowl, break up white chocolate. Add shortening. Heat for 30 seconds at 50% power in the microwave. Stir (even if nothing melted). Continue heating at 30 second intervals (50% power), stirring between each heating cycle, until chocolate is melted. Stir in 1/2 tsp. pumpkin pie spice.
Pour melted chocolate over cereal. Stir carefully, coating all the cereal.
Add half of the chocolate-coated cereal to the prepared Ziploc bag. Toss to coat everything, breaking up any chunks. After everything is mixed, spread out to dry.
Add remaining half of powdered sugar and remaining 1/2 tsp. pumpkin pie spice to Ziploc bag. Add remaining half of chocolate-coated cereal. Toss to coat everything, breaking up any chunks. After everything is mixed, spread out to dry.