1Spray the bottom of a 9x13-inch baking dish with cooking spray.
2Spread 1/2 of the peanuts into the prepared baking dish.
3Cook and stir sweetened condensed milk, peanut butter chips, and butter in a saucepan over medium heat until melted, about 5 minutes. Add marshmallows and stir until smooth. Pour mixture over peanuts and top with remaining peanuts.
4Refrigerate until chilled, 1 to 2 hours. Cut into bars.