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

In a mixing bowl, cream together unsalted butter and both sugars until light and fluffy (3-5 minutes). Mix in the egg and vanilla extract until combined.

In a separate bowl, whisk together all-purpose flour, Dutch-process cocoa powder, baking soda, and salt. Gradually add this dry mixture to the butter mixture, stirring until just combined.

Gently fold in semi-sweet chocolate chips into the brownie dough.

In another mixing bowl, cream together unsalted butter and both sugars until fluffy. Add the egg, additional egg yolk, and vanilla extract, mixing until well blended.

Combine all-purpose flour, baking soda, cornstarch, and salt in a different bowl. Slowly add this dry mixture into the butter and sugar mixture until just combined.

Divide both brownie and chocolate chip cookie dough into equal-sized balls (about 1 tablespoon each). Gently combine one brownie ball with one chocolate chip dough ball.

Place combined dough balls on baking sheets, ensuring space for spreading. Bake for 8-9 minutes or until edges are firm and centers look slightly underbaked.

Let cookies cool on baking sheets before transferring to a wire rack. Optionally, sprinkle flaky sea salt on top while warm.