Preheat the oven to 350°F and line a 9x13 baking dish with parchment paper or aluminum foil. Coat with cooking spray. Set aside.
In a large mixing bowl, combine the brownie mix, vegetable oil, water and eggs. Set aside.
Place all 30 Oreos into a large plastic bag and using a rolling pin, break into larger pieces. Some can be crumbled more than others. Divide the crumbled Oreos into two equal parts.
Fold one part of the Oreos into the brownie batter. Pour the batter into the prepared baking dish.
Top the brownies with the remaining crumbled cookies.
Bake for 23-26 minutes or until the center has set.
Remove and allow to cool fully.
Whisk together the powdered sugar, vanilla extract and milk until smooth. Drizzle over cooled brownies using the tines of fork, a pastry bag or a plastic bag with a corner snipped.
Cut into squares.
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