In saucepan,combine butter,brown sugar,salt and karo syrup. Boil for 1 1/2 minutes. Take off heat and add baking soda. Place chex mix and peanuts in a large paper bag. Add heated mixture. Microwave for 1 1/2 minutes, 3 times.( Shake bag between each cooking time.) Pour out on wax paper to cool. Separate pieces as it is cooling, and add M&M's.