Start by making the cookie dough layer. Beat the butter and light brown sugar together until light and fluffy. Be sure to scrape down the sides of the bowl to make sure all the butter is well blended.
1 cup butter, 1 ¼ cup light brown sugar
Add the egg and vanilla and mix until smooth.
1 egg, 1 Tablespoon vanilla extract
Add the flour, salt, baking powder and baking soda and mix into a smooth, thick dough.
2 1/4 cups flour, 1 teaspoon salt, ½ teaspoon baking powder, ¼ teaspoon baking soda
Stir the chocolate chips into the batter by hand.
1 cup chocolate chips
Spray a 9x9 baking dish with baking spray or brush it with melted butter.
Add the cookie dough to the greased pan and press it into the bottom of the pan, making it flat and even.
Make a layer of Oreos on top of the cookie dough. Try to cover the cookie dough completely, breaking the cookies as needed to make sure they fit.
Place the pan in the fridge while you make the brownie layer.
Preheat your oven to 350℉.
Sift the cocoa powder, flour, salt and baking powder together into a medium sized bowl. Set aside.
⅔ cup unsweetened cocoa powder, ½ cup flour, ½ teaspoon salt, ½ teaspoon baking powder
Beat the eggs, melted butter, sugar and vanilla together. Beat for about 5 minutes so the mix is thick. This will make the brownies have that nice, thin, crispy top.
¾ cup butter, 1 cup light brown sugar, 2 eggs, 1 Tablespoon vanilla extract
Stir in the dry ingredients until smooth.
Pour the brownie batter into the pan, over the top of the Oreo layer and shake the pan a little so it settle into a flat layer. Sprinkle walnuts on top, if using.
½ cup chopped walnuts
Bake the bars for 40 minutes. A toothpick inserted into the center will come out mostly clean- a little bit sticky is okay but if the toothpick is completely covered in batter, bake for a few more minutes
Let the bars cool for about 2 hours. They will slice easier when they are not hot.