For the Cookie Dough

1. Preheat your oven to 325 degrees Fahrenheit. Line a baking pan with parchment paper and spray with cooking oil to ensure the bars come out cleanly when fully cooled.
2. In a medium bowl, whisk all purpose flour, corn starch, baking soda, and salt together until combined.
3. In a large bowl with an electric hand mixer or in the bowl of your stand mixer with your paddle attachment, beat room temperature butter, light brown sugar, and granulated sugar until smooth and fluffy, about 2 minutes on medium-high speed.
4. Add vanilla extract, and eggs to the mixture, beating until smooth and fully incorporated. Scrape down the sides of the bowl as needed.
5. Slowly, add dry ingredients to the wet ingredients on low speed. Mix just until dry ingredients disappear into the wet. Add chocolate chips and mix until combined.
6. Place to the side until assembly.

For the Brownies

1. In a large bowl with an electric hand mixer or in the bowl of your stand mixer with your paddle attachment, beat eggs, light brown sugar, and granulated sugar on high speed for 1-2 minutes until light pale in color, greater in volume, and slightly foamy in texture.
2. Add melted butter and vanilla extract and mix until smooth.
3. Carefully, whisk all purpose flour, unsweetened cocoa powder, and salt into the batter. Whisk only until the dry ingredients disappear into the batter, as mixing past this point could lead to dry brownies.

For Assembly 

1. Alternate adding brownie batter and cookie dough batter to your prepared pan in about 1/4 cup increments.
2. If desired, top cookie dough batter with mini chocolate chips and/or chocolate chunks.
3. Bake at 325 for 30-35 minutes or until the top of the cookie dough is light golden brown in color, the batter does not shake when the pan is moved, and a toothpick inserted into the batter comes out with slightly moist batter. It should not come out clean or even with formed crumbs, but the batter should not look slimy or wet.
4. Allow the bars to fully cool before slicing.
5. Store, covered, at room temperature for up to 4 days.