Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper.

In a large mixing bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy.

Add the eggs one at a time, mixing well after each addition, then stir in the vanilla extract.

In a separate bowl, whisk together the flour, baking soda, and salt.

Gradually add the dry ingredients to the wet mixture, mixing until just combined. Fold in the chocolate chips.

In another bowl, combine the brownie mix and water until smooth.

To create the marbled effect, take a tablespoon of cookie dough and a tablespoon of brownie batter. Place them on the prepared baking sheet, slightly overlapping them.

Use a toothpick or a knife to swirl the two together gently.

Bake in the preheated oven for 10-12 minutes, or until the edges are golden brown.

Allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.