Line a 9x13 baking pan with foil or parchment paper. Coat with extra butter; set aside.
In a large pot melt the 8 tablespoons butter over low heat. Stir in the marshmallows and continue stirring until most of the marshmallows are melted. (I like to leave plenty that aren't fully melted for pockets of gooey marshmallow throughout the treats!)
Remove from heat and stir in the vanilla and salt. Stir in the cereal until completely coated.
Spoon into prepared pan and with well-buttered hands press into an even layer. Let cool at least 30 minutes before cutting.