Melt butter and peanut butter chips in a large saucepan over medium heat until smooth, then stir in condensed milk, vanilla extract and marshmallows, stirring until smooth and incorporated.
Generously grease a 9×13-inch baking dish with butter or non-stick spray, or line with parchment paper, then spread half of peanuts across baking dish.
Pour condensed milk mixture over peanuts, then sprinkle remaining peanuts on top.