In a heatproof bowl, over a pot of simmering water, melt the chocolate or Melt in the microwave at 30 secs intervals, stirring in between.
Line a 8 x 8-inch pan with 2 layers of aluminum foil, letting it hang over the sides.
Crush Oreo cookies to make 1 and ½ cups Oreo crumbs as shown or in a food processor and combine with melted butter.
Press into the bottom of the prepared pan and place in the freezer while you make the filling.
Beat together chilled whipping cream with ¼ cup powdered sugar until stiff peaks form.
In a large mixing bowl, beat together cream cheese with ¾ cup of powdered sugar until creamy.
Add thick hung curd / sour cream followed by the melted and cooled chocolate and mix until combined.
Fold in the whipped cream.
Spread the filling over the set Oreo crust and level and place in the refrigerator for a minimum of 4 hours. It's best kept overnight so that the cheesecake can set nicely.
Demold and serve with a drizzle of melted chocolate and whipped cream (optional).
Notes
You can also pour a layer of chocolate ganache over the filling and leave it to set. But the Oreo crust and the filling were rich and chocolaty enough for us.