Brownie Batter

1. In a bowl, place the melted butter, eggs, white granulated sugar, brown sugar, vanilla, and cocoa powder, and mix with an electric mixer until you obtain a smooth mixture. 
2. Add the dry ingredients—flour, baking powder, salt, and chocolate chips—and mix with a spatula until fully incorporated and the batter is smooth. 
3. Pour the brownie batter into the bottom of a square pan lined with parchment paper, and use an offset spatula to smooth the brownie layer. 
4. Place a layer of Oreo cookies on top of the brownie batter and press them down slightly.

Cookie Dough

1. In a bowl, place the melted butter with the white granulated sugar and brown sugar, and beat with an electric mixer for about 1 minute. 
2. Add the egg and vanilla extract, and whisk again to incorporate. 
3. Finish by adding the dry ingredients—flour, baking powder, salt, and chocolate chips—and gently fold them in with a spatula until the dough is smooth. 
4. Spread the cookie dough over the brownie layer in the pan, making sure to completely cover the Oreo cookies. 
5. Decorate with pieces of Oreo cookies and bake in a preheated oven at 180°C (350°F) for about 30 minutes, until golden on top but still slightly soft in the center. (Test by inserting a toothpick in the center; it should come out with brownie crumbs, not overly wet batter). 
6. Let the Oreo brookie cool completely in the pan, then place it in the fridge for 1 hour to make it easier to cut into 16 pieces with a large knife.