Preheat oven to 250°F (120°C). Line or lightly grease a jelly roll pan.
In a large bowl, combine 6 cups Cheerios, 2 cups peanuts, and 1 cup raisins.
In a small saucepan, combine 1/2 cup butter, 1 cup packed brown sugar, 1/2 tsp salt, and 1/4 cup light corn syrup. Heat over medium, stirring until the sugar dissolves. Bring to a boil and boil for 2 minutes, stirring frequently.
Remove the saucepan from the heat and stir in 1/2 tsp baking soda and 1 tsp vanilla (the mixture will foam). Stir until evenly combined.
Immediately pour the hot syrup over the Cheerios mixture. Stir gently but thoroughly until the cereal, peanuts, and raisins are evenly coated.
Spread the coated mixture in an even layer on the prepared jelly roll pan. Bake at 250°F for 15 minutes.
Remove from the oven and let the mixture cool completely on the pan. Once cooled, break into bite-sized clusters.
Add 1 1/2 cups M&M's to the cooled clusters and stir to combine. Store in an airtight container.