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Tip the flour into a mixing bowl and remove 2 tsp (the cocoa will replace it later). Add the butter and rub into the flour until the mixture resembles fine breadcrumbs. Sift in the cocoa and icing sugar, then, using a round-bladed knife, stir in the oil, egg yolk and 1½-2 tbsp cold water, until the dough comes together. Gently gather into a ball, then roll out on a lightly floured surface, big enough to fit a 20cm round x 3.5cm-deep loose-bottomed flan tin. Ease the pastry into the tin, leaving a slight overhang. Lightly prick the base with a fork, then chill for about 10 mins.
Heat oven to 190C/170C fan/gas 5. Sit tin on a baking sheet. Line the pastry with foil and fill with baking beans. Blind-bake for 15 mins or until set. Carefully lift out the beans and paper, then bake the pastry case for another 10 mins or until the base is cooked. Remove, carefully trim off the overhanging pastry with a sharp knife, to give the pastry a flat edge, then leave until completely cold.
To make the filling, put the chocolate in a large bowl that will fit over a pan of simmering water without touching it. Mix the cocoa, coffee and vanilla with the milk. Pour over the chocolate. Sit the bowl over a pan of gently simmering water, stir, then immediately remove pan from the heat, with the bowl of chocolate still over the water, stirring occasionally, to check when melted. Stir the melted chocolate – it will be quite thick. Stir in 2 tbsp boiling water and the chocolate will immediately thin down and become silky smooth. Take the bowl off the pan and leave to cool slightly.
Whisk the egg whites to stiff peaks, then whisk in the sugar until thick and glossy. Fold the créme fraîche into the cooled chocolate. Fold one-third of the egg whites into the chocolate mixture using a large metal spoon, then very 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, then spread out gently and evenly. Chill for about 3 hours, or overnight, before serving. Serve with a sifting of cocoa and half-fat créme fraîche, if you like. Please note, pregnant women, the elderly, babies and toddlers, and people who are generally unwell, should avoid eating raw or partially cooked eggs.