In a large mixing bowl, combine the rice cereal and brownie mix, stirring until evenly coated.

In a microwave-safe bowl, combine the peanut butter and chocolate chips. Microwave in 30-second intervals, stirring between each, until melted and smooth.

Pour the melted peanut butter and chocolate mixture over the rice cereal and brownie mix. Stir gently until all the cereal is evenly coated.

In a separate large zip-top bag, combine the powdered sugar, cinnamon, and pumpkin spice.

Add the coated cereal mixture to the bag, seal it, and shake until the mixture is well coated with the powdered sugar and spices.

Transfer the puppy chow to a large serving bowl and mix in the candy corn, pretzel sticks, mini marshmallows, and M&M's.

Serve immediately or store in an airtight container for up to one week.