Step 1: Preheat the oven to 250°F (120°C) for even caramelization.
Step 2: Line a large baking sheet with parchment paper.
Step 3: Melt the butter in a medium saucepan over medium heat, then stir in brown sugar and corn syrup. Bring to a boil and cook for 2 minutes without stirring.
Step 4: Remove from heat and stir in baking soda and vanilla; mixture will bubble.
Step 5: In a large mixing bowl, combine Rice Chex and cashews. Pour the caramel over and stir until coated.
Step 6: Spread the mixture on the baking sheet and bake for 1 hour, stirring every 15 minutes.
Step 7: Stir in M&Ms after removing from the oven and cooling slightly.
Step 8: Allow to cool completely before transferring to an airtight container.
