Preheat oven to 350 degrees
Line a 9x9 pan with parchment paper
Mix the brownie mix, vegetable oil, eggs and water together until fully combined. Pour half the brownie batter into the prepared pan. Set the rest aside.
To a pot over low medium heat melt the butter and peanut butter together. Once smooth add the marshmallows. Melt together.
Evenly pour the peanut butter caramel on top of the brownie batter. Move quickly becuase it gets harder to work with as it sets.
Top with the second half of the brownie batter. Use a spatula to spread out as evenly as you can.
**optional - add a few marshmallows on top, either use mini marshmallows or cut up large ones in fourths.**
Bake in the middle rack for 35-37 minutes.
Let completely cool before cutting.