In a BIG bowl measure out the Rice Krispies and ½ bag of marshmallows.
Brown the butter in a large pan, remove it from the heat and add the fluff.
Once that is almost incorporated, add 1 ½ bags of marshmallows (you might have to put it back on the heat to melt it all). Pour over the Rice Krispies and marshmallows and mix.
Press into a greased 9x13 pan.