Preheat oven to 350 degrees. Line a 9x13-inch baking pan with tin foil. Lightly spray the foil with nonstick baking spray.
Add the brownie mix to a large mixing bowl.
Add in the eggs, vegetable oil, and water. Stir until the eggs are incorporated.
Pour the brownie batter into the prepared baking dish.
Place 24 oreos on the top of the brownie batter. I did 6 rows of 4. Push them down into the batter.
Place in the preheated oven and bake for 25-30 minutes or until a toothpick that's inserted in the center of the brownie comes out clean. Cool completely before adding frosting.
Frosting
Remove the cooled brownies from the pan, using the foil overhang, and place the slab and foil onto a cookie sheet.
Add the ¾ cup of butter to a large mixing bowl.
Add in the powdered sugar, milk, salt, and vanilla. Beat with a hand mixer on medium until well incorporated.
Chop the remaining 6 oreos up into small pieces and beat them into the frosting until evenly distributed.
Spread the frosting over the cooled brownies with an offset spatula. Be sure to spread to all of the edges.
Place the brownies in the fridge to chill the frosting until firm.
Topping
Place the butter and chocolate chips into a small saucepan over low/medium heat until everything is melted and combined. Stir often with a spatula while the chocolate chips are melting.
Remove from the heat and pour the chocolate over the frosting and spread it evenly over the brownies. Refrigerate for at least an hour until the chocolate is completely set.
Take the brownies out and slice with a large knife, making quick cuts to prevent the top from cracking.