Grease a 9 x 13 pan with a little cooking spray or butter.
In a very large and deep pan (such as the Le Creuset 3.5 qt. Braiser), melt the butter on medium heat. Wait until it turns a light golden brown.
Add one bag of marshmallows to the butter, stirring constantly. Once that bag is somewhat melted, add the second bag, and turn the heat to low. Melt the marshmallows completely.
Add the vanilla and salt to the marshmallow mixture.
Keeping the heat on low, add about 4 cups of the Rice Krispies, stirring constantly. (I use two rubber spatulas to do this, and toss it like a salad.) Once the cereal is incorporated, add another 3 cups of cereal and toss. You will see the cereal and marshmallows start to get a little toasted. Finally add the last 3 cups of cereal, and remove from heat.
Transfer the mixture into the greased 9 x 13" pan. Wet your hands with cold water, and gently press down on the mixture until it is packed in the pan. (Be careful. It's hot.)
Let the treats sit for at least 30 minutes and cut into 12 squares.