Pour 1/2 of the peanuts into the bottom of a 9x13-inch baking dish.
Melt peanut butter chips and butter in a microwave-safe glass or ceramic bowl in 30-second intervals, stirring after each melting, for 1 to 3 minutes. Stir in sweetened condensed milk and microwave until heated through, about 30 seconds more.
Stir marshmallows into butter mixture until completely incorporated and melted. Pour marshmallow mixture over peanuts into the baking dish. Sprinkle remaining peanuts over the top, cover the peanuts with a sheet of waxed paper, and press down to fill the pan and create an even layer. Remove wax paper and cool completely before cutting into squares.
