Blitz cookies and chocolate into crumbs in a food processor, then add butter and salt until clumped.
Press the mixture into a 23cm tart tin and refrigerate for at least 1 hour to harden.
Whisk cornflour into milk in a small cup until smooth.
Heat cream, chopped chocolate, cocoa, coffee, sugar, vanilla, oil, and salt in a pan over low heat until melted.
Whisk in the cornflour mixture and stir over low heat for about 10 minutes until thickened.
Pour mixture into a jug and chill with damp parchment paper on the surface for 15 minutes.
Pour the filling into the tart base and refrigerate overnight to set.
Unmould the tart 10 minutes before serving by sitting it on a large jar to release the ring.
Slice and serve with crème fraîche.