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

Combine cold unsalted butter, brown sugar, and granulated sugar in a mixer and beat for 3 minutes until light and fluffy.

Add the room-temperature egg and vanilla extract, mixing until fully combined.

In a separate bowl, sift together cocoa powder, flour, baking soda, and salt, then mix into the butter-sugar mixture.

Fold in semi-sweet chocolate chips.

In a clean bowl, cream together cold unsalted butter with brown sugar and granulated sugar for about 4 minutes.

Add another room-temperature egg, egg yolk, and vanilla extract, mixing until smooth.

Whisk together flour, cornstarch, baking soda, and salt in another bowl, and add this to the cookie dough.

Stir in remaining semi-sweet chocolate chips.

Divide both the brownie dough and cookie dough into equal portions, rolling them into balls.

Take one brownie ball and one cookie ball, gently roll them together to form a single ball.

Place forming balls on lined baking sheets, spacing them 2 inches apart.

Bake for 8-9 minutes until edges are set and center looks slightly underbaked.

Cool on the sheets for a few minutes before transferring to a wire rack.