Preheat oven to 350°F. Grease and line with baking paper 1 9×9″ square pan. Set aside.
In a bowl, combine brownie mix, water, oil and eggs. Mix well.
For the filling, combine condensed milk and peanut butter.
Pour half of the brownie batter into prepared pan. Spread evenly on the bottom of the pan. Pour peanut butter mixture and spread evenly. Pour in the remaining brownie batter covering as much of the peanut butter mixture as much as possible.
Bake for 30-40 minutes until top of brownie is set and sides are starting to pull off from pan. Let cool completely before unmolding. Slice before serving.