1. Preheat the oven to 350°F. Grease a 9×13-inch pan and line the bottom and sides with parchment paper. Allow the butter to soften for at least 20 minutes.

Make the Cookie Dough

1. Combine the softened butter with the brown and granulated sugar. Beat on medium speed until well combined, 2-3 minutes.
2. Next add the egg, egg yolk, and vanilla extract and continue beating until well incorporated.
3. In a separate bowl, combine the flour, salt, and baking soda. Stir to combine and slowly add the dry ingredients into the dough, mixing on low speed until the dough starts to form. Before the dough thickens, add the chocolate chips. Beat until incorporated. Set aside.

Make the Brownie Batter

1. In a medium-sized mixing bowl, combine the granulated sugar with the oil, milk, and vanilla extract and beat together until well combined. Next, add the egg and beat just until combined, about 30 seconds or so.
2. Combine the dry ingredients in a separate bowl and stir to combine. Add the dry ingredients to the batter and mix just until combined. Fold in the chocolate chips with a spatula.

Assembly & Baking

1. Pour the prepared brownie batter into the bottom of the pan. Line the pan with 24 Oreos. Then take large pieces of the cookie dough and press it flat with your hands, then press it into the brownie batter. Continue to layer all the remaining cookie dough.
2. Bake at 350°F for 35-40 minutes. First, bake for 15 minutes, then cover the pan with foil to prevent too much browning on top. Around 30 minutes into baking, remove the foil. Test for doneness by inserting a toothpick into the center. The toothpick should be clean. The brownie layer will be fully baked through, the cookie dough may be slightly underdone but will continue baking as it cools. Cool completely before slicing.