Spray 9x13 pan with cooking spray.
In a pot, melt the butter and salt over medium heat. Once melted, add 4 cups of mini marshmallows and stir until completely melted.
Take the mixture off the heat and add in cereal and stir. Add in an extra cup of mini marshmallows and stir to combine.
Working quickly, pour half the mixture into the prepared pan and press down the mixture. Top with half of the chocolate bars. Then pour the rest of the mixture onto the chocolate and press on the remaining chocolate pieces.