2For the brownie layer: In a medium-sized sauce pan, melt the butter over medium high heat. Add the sugar and cocoa powder once the butter is melted. Whisk to combine and remove from heat. Add the salt, vanilla and eggs and continuously whisk until the eggs are combined. Add the flour and continue to mix. Set batter aside.
3For the cookie dough layer: Cream together the butter and sugars using a mixer. Add the eggs and vanilla, making ure to scrape down the sides of the mixing bowl. Add the flour, salt, baking soda and baking powder and mix on low until everything is incorporated. Fold in the chocolate chips. Set dough aside.
4For the assembly:
Line the bottom of a 9 x9 inch baking pan with aluminum foil and the spray with cooking spray.
5Layer half of the cookie dough on the bottom of the prepared baking pan, pressing down to form the bottom of the bars. Reserve the remaining cookie dough to be used to make cookies later in the day.
6Layer as many Oreos as you can on top of the cookie dough. No need to overlap. One single layer.
7Pour the brownie batter on top of the Oreo layer and make sure it's spread evenly on top. Bake for 30 - 35 minutes. Test with a knife to see if the center is done. If the knife comes out clean, then remove the pan from the oven and let the brownies rest before serving. If the knife comes out with batter still on it, allow the brownies to bake about 5 minutes more.