Preheat your oven to 350 degrees F. Lightly grease a 13 by 9-inch baking pan. This ensures your cake will come out easily when it’s done baking.
In a large bowl or the bowl of a stand mixer, combine the all-purpose flour, sugar, baking soda, baking powder, salt, melted butter, and egg. Stir until well blended. You should see a smooth, thick batter forming.
Add in the milk and mix until just incorporated. This mixture should be thick but pourable. Pat the mixture into your prepared pan and make sure it’s evenly spread out.
Now, in a stand mixer or using a hand mixer, beat the cream cheese until smooth. You want it nice and creamy with no lumps.
Add the remaining eggs and cocoa powder to the cream cheese mixture. Lower the speed of the mixer and add the powdered sugar. This will keep the mixture from flying everywhere!
Continue beating until all ingredients are well mixed. Slowly pour in the remaining melted butter and mix in the vanilla. The mixture should be smooth and glossy.
Spread this cream cheese filling over the cake mixture in the pan. Press 18 whole Oreos into the filling gently. Don’t worry if the filling spills over a bit; that’s part of the charm!
Bake for 40 to 50 minutes. Keep an eye on it; the center should still be a little giggly when finished. This ensures a moist, gooey cake. Let it sit for five minutes in the pan after baking.
While the cake is sitting, chop the remaining Oreos into pieces. In a microwave-safe bowl, melt the chocolate chips and peanut butter together. Stir every 30 seconds until melted and smooth. The aroma will be heavenly!
Once melted, evenly distribute the chopped Oreos over the top of the cake. Then, pour the melted chocolate mixture over everything, spreading it with a spatula. Make sure to push the chocolate into the Oreo crevices for the best results.
Refrigerate for 2 hours before serving. This helps the layers set beautifully. When ready to serve, let the bars sit at room temperature for about 15 minutes. Then cut them carefully, though the tops might crack a little.