• Preheat oven to 160º C/ 320º F.
• 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.

Crust:

• 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.

Filling:

• 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.

Chocolate Mousse:

• 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.