Preheat the oven to 350°F (175°C) and line a baking sheet with parchment paper.
In a large bowl, mix melted butter, granulated sugar, and brown sugar until smooth.
Add the egg, vanilla extract, and peanut butter. Mix until well combined.
Stir in flour, salt, and baking soda until a soft dough forms.
In a separate small bowl, mix the brownie mix, oil, and water until just combined.
Scoop cookie dough onto the baking sheet. Top each with a spoonful of brownie mix and swirl lightly using a toothpick or knife.
Bake for 10–12 minutes or until the edges are golden. Let cool on the pan for 5 minutes before transferring to a wire rack.