Preheat oven to 350 degrees. Lightly grease a 13×9×2-inch baking pan and set aside.
In a large bowl, combine brownie mix, eggs, oil and vanilla. Beat until combined. Stir in peanuts. Remove half of the brownie mix and set aside.
Spread remaining brownie mix into prepared pan.
Whisk together sweetened condensed milk and peanut butter in a medium bowl until smooth. spread peanut butter mixture evenly over the brownie mixture in the pan. Separate the reserved brownie mixture into pieces. Flatten each piece with your fingers and place on top of peanut butter mixture.
Bake in preheated oven for 25 to 30 minutes or until top is set and edges are lightly browned. Cool completely in pan on a wire rack.