Grease the inside of a casserole dish or pan with softened butter (for this recipe a 12 x 8 size casserole dish is used). Set aside.
In a large bowl, add the puffed rice cereal. Set aside.
In another large bowl, (one that can go in the microwave) add the softened butter. Make sure to use softened butter so that it will melt quickly along with the marshmallows when warmed in the microwave.
To the microwaveable large bowl, add all of the sour apple marshmallows that are in the package. The package generally has 23 sour apple marshmallows in it.
Warm the marshmallows and butter in the microwave for 40 seconds. The warmed marshmallows should look soft but not completely melted.
Then using a soft spatula, gently stir the marshmallow mixture just enough until it's creamy.
Be careful not to over-warm or over-stir the marshmallow mixture or it will seize up (be sticky and stiff).
To the bowl with the puffed rice cereal, add the melted marshmallow mixture.
Using a soft spatula or a large sturdy spoon, gently mix the ingredients together until the cereal is coated with the marshmallow mixture.
Transfer the mixture into the greased casserole dish. Use a soft spatula to press the mixture evenly into the dish. You can also use your hands with just a little water on them to press the mixture into the dish.