Preheat your oven to 350°F (175°C). This ensures that your brownies bake evenly.
In a large mixing bowl, prepare the brownie mix according to the package instructions. Typically, this will include adding water, vegetable oil, and an egg. Stir until just combined, and set aside.
In another mixing bowl, combine the softened cream cheese, egg, creamy peanut butter, and granulated sugar. Use a whisk or an electric mixer to blend until the mixture is smooth and creamy.
Pour half of the brownie batter into the prepared 9x13-inch baking pan, spreading it evenly. Then, carefully spoon the cheesecake mixture over the brownie layer, ensuring it covers the brownie base.
Sprinkle the chopped salted peanuts and the chopped Reese's Peanut Butter Cups over the cheesecake layer. Finally, dollop the remaining brownie batter over the top and gently swirl it in with a spatula for a marbled effect.
Place the pan in the preheated oven and bake for 30-35 minutes, or until the edges are set and the center still has a slight jiggle. Keep an eye on them to prevent overbaking.
Once baked, remove the brownies from the oven and let them cool in the pan for about 15-20 minutes. Then, transfer them to the refrigerator to cool completely before cutting into squares. This helps the cheesecake layer set up nicely.