Preheat your oven to 350 F and butter a spring form cake pan (9 inch)
In a food processor, blend the Oreos until they are finely ground. Pulse in the butter until they are moistened.
Pour the crushed oreos and butter into the cake pan and firmly press into the bottom and the sides (about 1/2 inch up the sides)
Bake the crust for about 10 minutes.
In a large bowl, beat the cream cheese on medium high until smooth. Beat in the sugar until fully combined and smooth.
Blend in the sour cream, vanilla, and salt. Next add in the egg and egg yolk until fully incorporated.
Fold in the chopped Oreos and set the cheesecake batter aside.
In a large bowl, blend all the ingredients until the batter is smooth. (Before mixing, place all ingredients in the bowl the order it is written.)
Take 1/4 of the brownie batter and spread onto the bottom of the cake pan (on top of the baked Oreo crust)
Using a cookie scoop, alternate scoops of cheesecake batter and brownie batter to create a marble effect.
Cover the top with foil and place in the center of the oven. (you can also fill a roasting pan with boiling water and set on the rack below the cheesecake to prevent the cake from cracking.)
Bake for 1 hour, then check to see if the center has cooked through. Bake an additional 10-15 minutes if not finished.
The brownie batter should still be a tiny bit gooey before taking out. It will finish baking. But you still want the brownie part to be gooey and fudgy.
When the time is up, open the oven door halfway and let the cheesecake sit inside the oven while it cools down.
Let completely cool before slicing. You can put it in the refrigerator to chill.
Drench the cheesecake in melted dark chocolate and top with more crushed Oreos.