Preheat the oven to 300°F.
Melt the butter, brown sugar and vanilla in a sauce pan with constant whisking on medium heat.
It should look life caramel when done.
In a VERY large bowl, add the Chex cereal, pretzels and peanuts
Pour the sauce over the mixture and toss gently until evenly coated.
Line two large baking sheets with parchment paper.
Divide mixture on the two baking sheets and spread in even layers.
Bake for 45 minutes and stir every 15 minutes.
Remove from the oven and allow it to cool.
Add the mixture to a large bowl, along M&M's and mix together.
Store in a large ziplock bag for 4-6 days for freshness.