Line a 9x13 pan with parchment and spray with cooking spray. Place the cereal in a very large bowl so you have room to mix.
Heat the butter in a heavy duty sauce pan over medium heat, swirling it occasionally, until it foams up and starts to form brown bits in the bottom. Watch it closely because if you burn it, the butter will taste bitter. You want golden brown flakes in the bottom, and as soon as you see those pull it off the heat. Add the marshmallows off the heat, and start to mix.
Mix until you have a smooth sauce, if the marshmallows aren't melting fully, place over low heat until they do. Stir in the salt and vanilla and quickly pour over the cereal. Mix quickly with a rubber spatula until it is well mixed.
Pack down into prepared pan (I use a greased back of a measuring cup for this) until you have an even layer. Allow sit at room temp until cool and firm, then cut into squares.