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

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

In a large mixing bowl, use an electric mixer to cream together the softened butter, brown sugar, and granulated sugar until light and fluffy.

Beat in the egg and vanilla extract until well combined.

Gradually add the flour mixture to the wet mixture, mixing until just combined.

Gently fold in the chocolate chip cookie dough and brownie batter. Do not overmix; the marbled effect is key.

Using a cookie scoop, drop rounded tablespoons of dough onto the prepared baking sheet, leaving space between each.

Place the baking sheet in the preheated oven and bake for about 20 minutes or until the edges are set and the centers are slightly soft.

Remove from the oven and let them cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.