Preheat your oven to 350°F (175°C). This ensures that your brownies will bake evenly.
Grease a 9x13 inch baking pan with 1 tablespoon of vegetable oil or line it with parchment paper for easy removal later.
In a large mixing bowl, whisk together the vegetable oil, granulated sugar, and cocoa powder until well combined. Add the eggs, vanilla extract, and water, mixing until smooth. Stir in the salt and flour until just combined; be careful not to overmix.
Pour the brownie batter into the prepared baking pan and spread it evenly. Bake for about 20-25 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs. Remove from the oven and let it cool slightly.
Once the brownies have cooled slightly, spread the marshmallow fluff evenly over the top while the brownies are still warm.
In a microwave-safe bowl, combine the unwrapped caramels and 3 tablespoons of milk. Microwave in 30-second intervals, stirring in between, until the mixture is smooth and creamy. Pour the caramel over the marshmallow layer, spreading it evenly.
In another microwave-safe bowl, combine the creamy peanut butter and 1 cup of chocolate chips. Microwave in 30-second increments until smooth, stirring after each interval. Drizzle this mixture over the caramel layer.
Sprinkle the remaining chocolate chips and chopped peanuts over the top of the peanut butter layer, pressing them in slightly to ensure they stick.
Return the brownies to the oven for an additional 10-15 minutes, just until everything is warm and melty. Keep an eye on them to prevent burning.
Once baked, allow the brownies to cool completely in the pan on a wire rack. Once cooled, lift them out using the parchment paper (if used) and slice them into squares.