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

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

Mix in the egg and vanilla extract until well combined.

Sift together the Dutch-process cocoa powder, flour, baking soda, and salt in a separate bowl, then fold into the wet mixture.

Gently fold in the semi-sweet chocolate chips.

In another bowl, cream the cold unsalted butter with granulated and light brown sugars.

Add the egg, egg yolk, and vanilla extract, mixing until combined.

Whisk together the flour, baking soda, and salt in a separate bowl, then gradually fold into the butter mixture.

Stir in the remaining chocolate chips.

Portion out equal amounts of both brownie and chocolate chip cookie doughs.

Press a brownie dough ball flat and place a chocolate chip cookie dough ball on top, rolling them into one cohesive ball.

Place the dough balls on the prepared sheets, spaced about 2 inches apart.

Bake for 8-9 minutes until edges are set and the centers are slightly puffy.

Let cool on baking sheets for 5 minutes before transferring to wire racks.