Pour the Rice Krispies into a very large wok, reserving the box and bag. (If you don't have a wok, pull out the largest pot you've got.) Place over medium-high heat and cook, stirring frequently and adjusting the heat as needed, until the cereal has turned slightly darker brown and tastes toasted. Transfer to a large bowl.
Place the butter in the wok and return to medium heat. Cook, stirring and swirling the pan frequently, until the butter melts, foams, and turns light golden brown.
Reduce the heat to medium-low and stir in the marshmallows. Cook, now stirring with a rubber spatula, until melted and smooth. Stir in the salt.
Reduce the heat all the way to low and add the Rice Krispies back to the wok. Stir gently until all of the cereal is coated with the marshmallow mixture, then remove from the heat.
Spray the inside of the now-empty cereal bag with non-stick oil spray, then carefully pack the Rice Krispie mixture into the bag. Let cool before eating straight from the box.