In a large bowl, add both types of cereal. Set aside.
To a microwavable medium size glass bowl, add the marshmallows and butter.
Heat the marshmallows and butter in the microwave just enough so that the marshmallows and butter start to melt (about 2 minutes). Stir the marshmallows and butter together until smooth.
Add the marshmallow mixture to the bowl of cereal and gently stir all ingredients together until the cereal is coated.
Coat a casserole dish or pan (13" x 9" x 2") with non-stick cooking spray and then add the marshmallow and cereal mixture to it.
Use a buttered spatula, wax paper, or add water to your hands to gently press the mixture evenly into the pan. Let cool.
Cut into 12 squares and serve.