Line a 7 inch/ 18 cm spring form pan with a circle of parchment paper. Wrap very well the pan with 2-3 sheets of aluminium foil.
Crush the oreos to crumbs in a food processor or with a rolling pin in a plastic bag.
Place the crumbs in a bowl and add the melted butter. Mix very well and press the mixture to the bottom of the prepared pan. Place the pan in the fridge.
Combine the cream cheese and the brown sugar in a large mixing bowl/ the bowl of a stand mixer, fitted with a paddle attachment. Mix very well with the mixer at a high speed, until combined. Add the cocoa powder and mix again until uniform.
Add the eggs one by one, mixing at a lower speed, until smooth. Scrape the sides of the bowl, if needed.
At the end gradually add the melted and slightly cooled chocolate, mixing again at a low speed, until combined.
Pour the filling into the crust in the pan. Smooth it out with a rubber spatula.
Place a large pan full of cold water in the oven. Place the cheesecake inside the pan.
Bake for about 60 to 75 minutes, or until the cheesecake is stable and just a small part in the centre jiggles slightly when the pan is shaken.
Let the cheesecake cool until room temperature. Refrigerate for at least 6 hours or overnight, before topping and serving.
Whip the cream in a large mixing bowl. Make sure it is fluffy but not too thick.
Add some cream to the melted and cooled chocolate (not the other way around) and mix energetically to combine. Repeat the procedure 2-3 times and then fold the whole chocolate into the cream.
Top the cheesecake with the chocolate mousse. Optionally sprinkle pistachio nuts on top.
Take the filling ingredients our of the fridge about 30 minutes before starting to prepare the cheesecake.
Store the cheesecake 4-5 days covered in the fridge.