With your mixer combine brownie mix, oil and eggs until incorporated and smooth.
Stir in chopped peanuts
Divide batter in half.
Press half batter into bottom of prepared pan (it will be a thin layer) and set the other half to the side.
In a large bowl combine peanut butter and sweetened condensed milk with whisk.
Pour mixture over the bottom layer of brownie.
Drop the remaining brownie mix on top of peanut butter mixture, pressing each spoonful lightly into peanut butter.
Bake for 25 minutes until just set.
Let cool completely before cutting into squares.
Making these right now for the third time. I leave out the peanuts (mostly because I’m too lazy to chop up peanuts or pay for them to be pre-chopped) and they’re still fan-fucking-tastic. So delicious.