Slice butter into pats, as this helps it melt more quickly and evenly. Put into a large pan on medium/low heat. Stir constantly.

Add marshmallows. Some Rice Krispie Treat recipes call for the bigger bag of marshmallows (16 oz.) I use the 10 oz. because it's less sugar that way, sticks just fine, and it's really unnecessary if you're using any sugary cereals in your mix.

When marshmallows are mostly melted into butter, add the cereal. Smooth until well combined.

Press into a 9x13 inch casserole dish.

Watch that "leftover" cereal disappear! :)