Preheat your oven to 350° F. Line a 9x13 rectangular light metal baking pan with foil, ensuring the foil extends over the edges. Give the foil a light coating of cooking spray and set it aside.
To kick off, let's create the cookie dough layer. In the bowl of a stand mixer, use the paddle attachment to cream together the butter and sugars until the mixture is creamy and smooth – this should take about 1 minute. Incorporate the egg and egg yolk one at a time, beating well after each addition. Follow this up with the addition of vanilla extract. Finally, introduce the baking soda, cornstarch, salt, and flour, mixing thoroughly. Toss in the chocolate chips and give it a good stir by hand.
Transfer the cookie dough into the prepared pan, using your hands to press it into an even layer. Now, it's time to arrange the Oreos in a neat grid pattern – envision 6 rows with 4 Oreos per row, totaling 24 Oreos. Set this delightful layer aside.
As the Oreos settle into their cozy cookie bed, let's focus on the brownie layer. In a medium saucepan over medium-low heat, melt together the butter and chopped chocolate bar, stirring until you achieve a velvety concoction. Remove the pan from the heat, let it stand for 5 minutes, and then stir in the granulated sugar, beating it well. Quickly add in the eggs and vanilla extract, mixing with vigor. Conclude this phase by introducing the flour, cocoa powder, and salt, ensuring everything blends seamlessly.
Pour the brownie batter generously over the Oreo layer, making an effort to spread it to the edges of the pan, enveloping the Oreo goodness completely.
Allow this delightful creation to bake for 35-45 minutes or until a toothpick, when inserted near the center, comes out clean or with moist, but not wet, crumbs. Once the baking is complete, exercise patience and let the bars cool in the pan entirely before you even think about cutting them into squares. This ensures a perfect, cohesive treat for your indulgence.
