Follow the directions on the brownie mix box to make the brownie batter. Once done, pour 3/4 of the mixture into the pan. Smooth it out.
Grab a bowl and mix the powdered sugar, peanut butter, egg, and vanilla. It should be smooth. Spoon that mixture over the top of the brownie batter and then smooth it out.
Add the rest of the brownie batter over the top of the peanut butter layer. Drop it in spoonfuls, and then use the spoon to swirl everything together carefully.
Sprinkle the chocolate chips and chopped peanuts on top.
Put the pan in the oven to bake for 30-35 minutes.