Makes 8 slices
THE PASTRY
plain flour 140g + extra for dusting
butter 50g, cubed
cocoa powder 2 tsp
icing sugar 1 tbsp (try Bluebird available at gourmet stores)
rapeseed oil 1 tbsp (try La Palisse available at gourmet stores)
egg yolk 1 medium
half-fat crème fraîche to serve (optional)
THE FILLING
dark chocolate 100g, very finely chopped (try Morde or Valrhona available at gourmet stores)
cocoa powder 1 tbsp + extra 1/2 tsp for sifting
coffee granules 3/4 tsp
vanilla essence 1/2 tsp
semi-skimmed milk 2 tbsp
egg whites 2 medium
dark muscovado sugar 2 tbsp (try Waitrose available at gourmet stores)
half-fat crème fraîche 85g (try President available at gourmet stores)
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 1 1/2-2 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 tin. Ease the pastry into the tin, leaving a slight overhang. Lightly prick the base with a fork, then chill for about 10 minutes.
Heat the oven to 190°C. Sit the tin on a baking sheet. Line the pastry with foil and fill with baking beans. Blind-bake for 15 minutes or until set. Carefully lift out the beans and paper, then bake the pastry case for another 10 minutes or until the base is cooked. Remove, carefully trim off the overhanging pastry with a sharp knife, to give the pastry a flat edge, and 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 crème fraîche into the cooled chocolate. Fold one-third of the egg whites into the chocolate mixture using a large metal spoon, hen 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 halffat crème fraîche, if you like.