Preheat the oven to 350º.

Prepare a 9x9" square baking pan by lining it with two sheets of parchment paper, criss crossing. Allow the paper to hang over the sides for easy removal. Spray with cooking spray and set aside.

To make the chocolate chip cookie dough:Add the brown sugar and granulated sugar to a medium sized bowl and add the melted butter. Whisk until well combined.

Add in the egg and the vanilla and mix until fully incorporated.

Stir in the flour, baking soda and salt, and mix until almost fully combined, but streaks of flour are still showing.

Add the chocolate chips and stir gently until the batter is just mixed and no streaks of flour remain.

Pour into the prepared pan and smooth out into an even layer with a spoon or your hands.

Next, add the Oreos in an even layer on top of the cookie dough.

To make the brownie batter:Add the chocolate chips and the butter to a heatproof bowl. Set the bowl over a small saucepan of barely simmering water--not boiling!

Melt the chocolate chips and butter slowly, stirring frequently until the mixture is completely melted and smooth.

Remove the chocolate mixture from the heat and whisk in the cocoa powder until smooth. Set aside.

In a separate bowl, whisk together the eggs, sugar, vanilla, and salt until well combined.

Add a couple of spoonfuls of the warm chocolate mixture to the egg and sugar mixture and whisk to temper the eggs.

Pour the remaining chocolate into the egg mixture and whisk until well combined.

Add the all purpose flour and stir with a wooden spoon or rubber spatula until just mixed.

Pour the brownie batter over the cookie dough Oreo layer. Use a rubber spatula or a spoon to spread the brownie batter out evenly. Make sure the Oreos are completely covered.

To Finish The Brownies:If adding the pretzels, add pretzels to the brownie batter layer in even rows.

Bake at 350º for 35-40 minutes until a toothpick inserted comes out with fudgy crumbs.

Allow the brownies to cool for at least two hours and cut into squares.