Preheat oven to 325F. Line a 9" x 9" baking pan with parchment paper, set aside. Place butter, sugar, salt and cocoa powder in a large microwave-safe bowl and microwave for about 1 minute and 30 seconds, or until butter is melted. Mix everything together until smooth. Stir in eggs and vanilla until combined. Add flour and mix. The batter will be very thick and glossy. Pour the batter into your prepared baking pan and smooth it out. Bake for 22-25 minutes. Remove from oven and let cool in the pan. While brownies are cooling, prepare the truffle layer. In a large food processor, add Oreo cookies and pulse to a fine crumb. Add cream cheese and pulse until a dough-like consistency forms and you can't see the cream cheese. Once brownies are completely cool, take chunks of the truffle mixture and pack it onto the brownie in an even layer. In a small bowl, melt the chocolate chips and butter at 30-second intervals in the microwave until completely smooth and pourable. Pour the chocolate over the truffle layer and spread it out into a smooth and even layer. Place into the refrigerator for 20 minutes to allow the top to set up properly before cutting.