Line a 9x13 inch baking pan with non-stick foil.
Combine butter, chocolate chips and sweetened condensed milk in a large saucepan and melt over medium low heat.
Meanwhile, combine peanuts and marshmallows in large bowl. Stir in chocolate mixture. Spread in prepared pan.
Chill 2 hours or until firm. Remove from pan by lifting edges of foil.
Peel off the foil and cut into squares.
Alternatively, you can make the recipe in the microwave. Heat the condensed milk and butter for 1 minute in a microwave-safe bowl. Add the chips and stir well. Microwave for another 30 seconds, stir until chips are melted and smooth. Then stir in peanuts and marshmallows. Press into the lined pan. Let set, then cut