Melt chocolate chips, butter and condensed milk in a double boiler. Alternatively you can use a large glass bowl on 50% power and melt mixture in the microwave in 30 second intervals stirring in between cook times.
Once melted allow to cool slightly, then stir in peanuts and marshmallows.
Pour mixture into lined pan and spread out so it's the same thickness all the way across.
Chill for 2-3 hours until chocolate hardens, then cut into squares and enjoy.