Preheat your oven to 350 degrees F or 320 fan forced (180 degrees C or 160 fan-forced). Melt Unsalted Butter (145 gram) and Dark Chocolate (175 gram) in the microwave.
In your mixing bowl, add Egg (2), the yolk of the Egg (1), Brown Sugar (145 gram), Vanilla Extract (2 teaspoon) and Salt (1/2 teaspoon).
Whisk on high for a good 5 minutes until pale and double in volume.
Sift in Unsweetened Cocoa Powder (1 tablespoon) and All-Purpose Flour (4 tablespoon).
Add in butter and chocolate mixture. Fold in gently.
Fold in half of the Oreo® Chocolate Sandwich Cookies (65 gram).
Transfer into a 7 by 7 inch square pan lined with greased-proof paper. Drop the pan lightly to release the air bubbles. Sprinkle the rest of the crumbled Oreo® Chocolate Sandwich Cookies (65 gram).
Bake it in the oven for 45 to 55 minutes.
Let it cool completely in the pan before transferring.
Cut the brownies into small squares, transfer into your platter and dust with Powdered Confectioners Sugar (to taste).