Melt the white chocolate: In a microwave-safe bowl, melt white chocolate chips in the microwave. Heat them in 15-second intervals, stirring in between, until the chocolate is completely melted and smooth.
Coat the Chex cereal: Line a large cookie sheet with parchment paper or a silicone baking mat. Pour the rice Chex cereal onto the prepared sheet.
Add the pumpkin pie spice: Once the white chocolate is melted, add the pumpkin pie spice to the bowl and stir until well combined. The pumpkin pie spice will infuse the white chocolate with the delightful flavors of pumpkin pie.
Coat the cereal: Pour the pumpkin-infused white chocolate over the Chex cereal. Gently toss and stir until all the cereal is evenly coated with the pumpkin-spiced chocolate. Be careful not to crush the cereal pieces.
Coat the cereal with powdered sugar: Sprinkle powdered sugar over the coated Chex cereal. Toss gently to ensure all the grain is covered with sweet and spiced powdered sugar.
Allow the mix to set: Let the Pumpkin Pie Chex Mix sit until the white chocolate coating has set. This may take about 30 minutes, depending on the room temperature.
Add the finishing touch: Once the mixture has hardened, add the colorful and festive touch of M&M's. Gently mix them into the Chex mix, creating bursts of color and sweetness.
Serve and enjoy: Transfer the Pumpkin Pie Chex Mix to a serving bowl or individual snack bags. It's now ready to be enjoyed! The combination of pumpkin-infused white chocolate, powdered sugar, and the delightful crunch of Chex cereal will surely be a hit.