Start by roughly breaking the Oreos and chocolate into pieces. Add these pieces to a food processor and blitz until you have crumbs. Add the butter and salt and blitz again until the mixture starts clumping together.
In a 23 cm/5 cm deep tart tin (with a loose bottom) with your hands press the Oreo & chocolate mixture until the sides and the base are even. I found it was easier to do this by pushing the mixture in the centre of the tart towards the sides. This ensured that the mixture was spread evenly.
Place in the fridge for at least 2-3 hours.
For the Filling:
Finely chop the chocolate. Set aside.
Pour cornflour into the milk and mix well until combined.
In a medium sized saucepan, pour the heavy cream. Add the sieved cocoa, instant coffee powder, sugar, vanilla extract, olive oil and salt. Heat over medium heat while gently whisking so as to get rid of any lumps.
As soon as the chocolate has completely melted, take the saucepan off the heat, and add in the cornflour milk mixture and mix until smoothly incorporated.
Put the saucepan back on medium heat, and keep stirring with a wooden spoon until the mixture thickens. This will take about 10 minutes. Nigella suggests to take the pan off heat every few minutes so as to avoid bringing the cream to a boil.
The chocolate filling is ready when it becomes so thick that it coats the back of the wooden spoon, and if you run your finger through it the line should stay.
Pour the chocolate filling into a bowl or jug from which it is easy to pour. Cover with a wet piece of parchment paper and place in the fridge for about 15 minutes.
After 15 minutes pour the chocolate filling onto the Oreo crust and make sure it is evenly spread and smooth. Place in the fridge for overnight.
Take out of the fridge 10 minutes before serving, but make sure to take it out of the tart tin right away.
Both the filling and crust can be made 1 day ahead and leftovers can be kept for 4-5 days.