In a medium bowl, whisk together the cocoa powder, flour, salt, and baking powder.

In a large bowl, whisk together the melted butter and sugar until smooth. Add the eggs and vanilla extract, mixing until combined. Gradually add the cocoa mixture, mixing just until combined. 

Pour the batter into the prepared pan and spread it evenly.

Whisk together the flour, baking soda, and salt. Set aside.

In a separate bowl, cream the softened butter, granulated sugar, and brown sugar together until light and fluffy. Mix in the egg, egg yolk, and vanilla extract.

Gradually add the flour mixture, mixing until just combined. Fold in chocolate chips.

Drop small spoonfuls of cookie dough evenly over the brownie batter. Gently press the dough down slightly to adhere to the brownie layer but don’t spread it out completely.

Bake for 30–35 minutes, or until the brownie layer is set but still slightly fudgy in the center. A toothpick inserted into the brownie part (not the cookie dough) should come out with a few moist crumbs.

Let the brookies cool completely in the pan before cutting into squares.