Preheat oven to 300 degrees F. Line a large rimmed baking sheet with heavy duty foil and spray lightly with non-stick cooking spray.
Pour popped popcorn out onto foil lined baking sheet. Pick out any kernels. Add Crispix and pretzels and spread out evenly with popcorn.
Melt butter in a large, heavy saucepan over medium heat. Add brown sugar, corn syrup, and salt. Bring to a boil, stirring constantly, and then allow to boil for two minutes, stirring occasionally. Remove from heat and use a spoon to drizzle the mixture over the popcorn mixture in an even stream. Toss the popcorn mixture lightly to distribute and then spread it back out in an even layer.
Bake for 10 minutes. Use a spatula to stir the mixture and bake for an additional 10 minutes. Remove from oven and stir once more. Spread the mixture back out in an even layer and allow to cool for 10 minutes or until cool enough to handle. Break apart and allow to cool completely. Add peanut M&M's and toss to combine.
Store in an airtight container or zippered plastic storage bags.