Line an 8x8" or 9x9" square baking pan with foil, extending the sides of the foil over the edges of the pan. Spray lightly with cooking spray; set aside.
In a large saucepan, whisk together the evaporated milk, chocolate chips and marshmallow fluff until smooth. While it melts down, in a large bowl, combine the powdered sugar and Oreo crumbs. Pour the chocolate/mallow mixture over the Oreo crumb mixture and stir to combine. Mixture will be thick and sticky.
Press the mixture into the prepared pan in an even layer, using an offset spatula if needed. Refrigerate while preparing the cookie dough topping.
In the bowl of a stand mixer, cream together the butter and brown sugar, about 1 minute. Add in the vanilla and heavy cream, beating well. Lastly, add in the salt and flour, until combined and a soft dough forms. Fold in the mini chocolate chips.
Spread the cookie dough mixture evenly over the brownie base, gently smoothing it out into an even layer. Refrigerate while you make the ganache.
In a medium bowl, combine the chocolate chips and heavy cream. Microwave on HIGH power for about 20-25 seconds. Stir, then microwave for another 10-15 seconds or until smooth and melted. Pour over the cookie dough layer and smooth out the top. Top with additional mini chips, if desired. Refrigerate for at least 2 hours to firm up.