1. Prepare a 9-inch springform pan by lining the bottom with parchment paper and line the sides with either parchment paper or a cake collar. Set aside.
2. Prepare the mousse: Place your mixing bowl in the freezer or refrigerator for 5-10 minutes before starting.
3. Grind 10 Oreos into a fine crumb using a food processor or blender. Set aside.
4. Spoon the mascarpone cream into your bowl and beat on a medium-low speed while slowly pouring in the heavy cream, allowing the mascarpone cheese to turn to a liquid consistency, then add the remaining heavy cream.
5. Once all the cream has been added, increase the mixing speed to high (speed 8-10) and beat it until soft peaks form.
6. Next add the vanilla extract, powdered sugar and Oreo crumbs. Beat until well combined, and mixture reaches stiff peaks.
7. Divide the mousse between two large piping bags. Cut the tip of the piping bags off, about ¾” up from the bottom.

To Assemble:

1. In a small bowl, combine the coffee and milk, stir to combine.
2. Pipe a very thin layer of mousse on the bottom of the pan and spread evenly.
3. Each layer will have 19 Oreos. Quickly dunk each Oreo in the prepared bowl of coffee &milk. Starting with the outside edge, place the cookies just inside the edge of the pan and work your way into the middle so they are tightly fit.
4. Starting on the outside edge, pipe the mousse in an even layer working your way towards the middle Then use an offset spatula to smooth out the mousse, creating a nice even layer.
5. Repeat this process two more times. There are 3 layers of Oreos and 4 layers of mousse. The top layer may be the thinnest. Cover and refrigerate for at least 4 hours. Prepare the topping prior to serving. Dust the top with cocoa powder if desired.
6. Make the topping: Place the mixing bowl and whisk attachment in the freezer for 5 to 10 minutes to chill. Pour the heavy whipping cream into the chilled bowl and use an electric mixer to beat the heavy cream at medium-high speed until soft peaks start to form. Slowly add the powdered sugar and continue beating at high speed until stiff peaks form. Pipe a border around the outside edge. If you have any leftover Oreos, you can use them for garnish or crush them up and sprinkle them on top. Keep refrigerated.