Begin by browning the butter for the chocolate chip cookie layer. Place the butter in a saucepan over medium heat, allowing it to melt and then cook until it turns a golden brown color.

Once it has browned, remove the pan from the heat and let it cool in the refrigerator for about an hour.

After the butter has cooled, beat it together with both types of sugar until the mixture is light and fluffy.

Add in the egg and the vanilla extract and mix until fully combined.

In a separate bowl, combine the flour, baking soda, and kosher salt. Gradually add this dry mixture to the butter-sugar mixture, mixing just until everything is combined.

Fold in the chocolate chips gently to ensure even distribution without over-mixing.

Prepare the brownie layer by melting the bittersweet chocolate along with the additional butter together.

Stir until smooth, then let the mixture cool slightly.

In another bowl, whisk together the egg, the additional egg yolk, both sugars, and vanilla extract.

Add the slightly cooled chocolate mixture to this, blending well.

In a separate bowl, mix together the flour, cocoa powder, baking powder, and kosher salt.

Carefully fold this dry mixture into the chocolate mixture, combining until just mixed.

Preheat your oven to 350°F and prepare a pan by greasing it with baking spray and lining it with parchment paper.

Pour half of the brownie mixture into the prepared pan. Smooth it out slightly before adding on half of the cookie dough.

Alternate layering the brownie and cookie mixtures, ensuring an even distribution of both.

Once the layers are assembled, level the top gently with a spatula.

Place the pan in the preheated oven and bake for approximately 25 minutes.

You'll know it's ready when the edges are golden brown, and a toothpick inserted in the center comes out mostly clean with a few moist crumbs.

After baking, remove the pan from the oven and allow it to cool at room temperature.

For best results, chill it in the refrigerator for an hour before slicing into squares to serve.