Grease and line a deep, loose bottomed tart tin.
Tip the flour into a mixing bowl along with the butter and rub into the flour using your fingers until the mixture resembles fine breadcrumbs.
Sift in the cocoa powder and icing sugar then, using a round bladed knife, stir in the oil, egg yolk and 1½-2 tablespoons of cold water, until the dough comes together.
Gather into a ball, then roll out on a lightly floured surface.
Ease the pastry into the tin, leaving a slight overhang.
Lightly prick the base with a fork, then chill for 10 minutes.
Preheat the oven to 190°C / 375°F / Gas mark 5.
Line the pastry with greaseproof paper and fill with baking beans.
Blind bake for 15 minutes.
arefully lift out the paper and the beans.
Bake the pastry for another 10 minutes.
Remove and carefully trim off the overhanging pastry with a sharp knife, then leave to cool completely.
For the filling, put the chocolate in a large bowl that will fit over a pan of simmering water without touching it.
Mix in the cocoa, coffee granules and vanilla with the milk.
Stir until everything has melted then immediately remove the pan from the heat.
Add in 2 tablespoons of boiling water which will make the milk mixture thinner.
Whisk the egg whites to stiff peaks, then whisk in the sugar until it looks glossy.
Fold in the crème fraiche into the cooled chocolate.
Then, using a large metal spoon, fold one-third of the egg whites into the chocolate mixture then gently fold in the remaining whites, a third at a time, until evenly mixed in.
Remove the pastry case from the tin and place on a serving plate.
Spoon the filling into the case and level.
Chill for 3 hours, or overnight, before serving.
Serve with a sifting of cocoa powder.