Preheat oven to 275 degrees. Line baking sheet with parchment paper.
Mix Chex cereal and pecans in large bowl.
In a separate bowl, mix brown sugar, pumpkin pie spice, vanilla, and butter.
Pour melted butter mixture over cereal and pecans. Stir until evenly coated.
Spread mixture on baking sheet. Bake for 45 minutes, stirring every 15 minutes.
Spread snack mix on wax paper to cool.
Add M&M'S and toss to combine. Store in airtight container.