Sprinkle 2/3 jar peanuts on bottom of 9x13" pan sprayed with non-stick spray; set aside.
Melt chips, butter, and sweetened condensed milk together.
Remove pan from heat; fold in marshmallows. Stir until well coated and melted only enough to swirl...do not over or under cook. ("Perfection" on this step will look almost like marshmallow creme swirled between layers of ooey-gooey peanut butter...barely a whole marshmallow in sight.) Press into pan over peanuts.
Press remaining peanuts on top. Refrigerate until serving for easiest cutting.