Heat the oven to 350° F. Line a 9x13 cake pan with parchment paper, or spray with cooking spray, and pour the brownie batter inside.
In a large mixing bowl, combine the dry brownie mix with the eggs, oil, and coffee (or water), stir until combined.
Pour the prepared brownie batter inside of the prepared baking pan.
Put the peanut butter into a small bowl and microwave for 30 seconds to melt it slightly.
After that, use a spoon to drop dollops of the peanut butter over the top of the brownie batter. Using a toothpick or knife, swirl the peanut butter throughout the batter. Be careful not to overmix the batter, or the swirls will blend into the batter.
If desired, sprinkle the top of the brownie batter with peanuts or another type of nut of your choice.
Bake for 20-25 minutes, or until a toothpick comes out clean.
Allow the brownies to cool slightly before removing them from the pan. Enjoy!