Line a 9 X 13-inch pan with aluminum foil, allowing 2-3 inches to extend over long sides. Spray generously with nonstick cooking spray.
Measure out 4 cups of marshmallows and set aside and reserve.
In a large saucepan, over low heat, melt butter (10 tablespoons). When the butter is melted add the marshmallows (excluding the 4 reserved cups) and continue to cook, stirring constantly, until the marshmallows are just melted. Remove from heat and stir in vanilla (1 1/2 teaspoons) and salt (1/2 teaspoon). Gently fold in cereal and coat evenly, then fold in the reserved 4 cups of marshmallows.
Spoon the mixture into the prepared pan and gently press into an even layer. Let the mixture come to room temperature and set. Then cut and serve.
Notes
Light touch: To create the requisite chewy gooey texture in these bars, whatever you do, do NOT press the mixture firmly into the pan. That seems to be how everyone made these treats originally, but doing so makes them too hard, crushes the cereal pieces, and ruins the delicate, gorgeous, fluffy texture that takes these Rice Krispie Treats from ordinary to extraordinary!