Rub a very large, light colored stainless steel saucepan or Dutch oven with butter to prevent sticking. Set over medium heat and melt the butter until it just starts to brown. This will happen at a different rate of speed depending on your stove. If it starts browning too quickly, remove from heat, swirl the pan, and put the butter back on at a very much reduced heat. Be careful not to burn it.

Stir in the marshmallows and reduce heat to low. Stir marshmallows until they begin melting. When melted, add the peanut butter (if using) and cereal and stir until coated.

Transfer mixture to pan and press down with a piece of greased foil or parchment. Let the treats cool completely then lift them out of the pan, place on a cutting board and score into giant size squares.