Prepare brownies as directed on pacakge, bake, and cool completely.
Once brownies are cooled, combine chocolate chips and sweetened condensed milk in a microwave safe bowl and heat in 30 second intervals, stirring well in between, until smooth. Stir in vanilla.
Fold marshmallows and peanuts into the chocolate until well combined. Scoop and spread on top of brownies into an even layer.
Allow brownies to set for 30 minutes before cutting into squares.