Gather your ingredients before you start to make the cereal bars. Prepare a 9x13" pan with non-stick cooking spray. Melting the marshmallows requires close attention to avoid burning.
Put the butter into a heavy pot on your stove top, and begin to melt it at medium heat.
Add the marshmallows and peanut butter. Use a spoon coated with non-stick spray to stir these items around to avoid scorching. This takes about five minutes; you may adjust the heat up just a notch if you find they aren't melting for you.
After the marshmallow, butter, and peanut butter mixture is smooth, stir in the Cheerios™
Turn off the stove. Stir in the chocolate chips and fold them in until evenly distributed.
Turn into the prepared 13x9" pan and smooth out the mixture. Cover and put the cereal bars in the fridge for about twenty minutes; they will become firm enough to cut. use a sharp knife to cut into cereal bar squares and serve.