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Make chocolate pastry by pulsing the pastry, 2 tbsp cocoa and the vanilla in a food processor. Roll out on a lightly floured work surface until it is the thickness of a £1 coin and use to line a 22-23cm loose-bottomed tart tin. Chill for 30 mins.
Heat oven to 200C/180C fan/gas 6. Scrunch up a sheet of baking parchment, then flatten it out and press gently over the pastry. Pile in baking beans and blind-bake for 15-20 mins until the pastry is biscuity. Remove the parchment and beans and bake for 5 mins more until cooked through. Brush with egg white and return to the oven for 2 mins; this will help to keep the base crisp. Reduce oven to 180C/160C fan/gas 4 and spread the jam over the base of the tart.
Melt the chocolate and butter in a microwave, in short bursts on High, stirring in between, or in a bowl over – but not touching – a pan of barely simmering water. Whisk the eggs, egg yolk and sugar with an electric whisk for a full 10 mins until pale and thick. Take your time when beating the eggs and sugar - they should be mousse-like. Sieve the flour and remaining cocoa and fold in, together with the melted chocolate, using a big metal spoon.
Scrape into the tin and scatter over the cherries. Bake for 20-25 mins until the filling has slightly puffed but is still slightly wobbly when you shake it. Chill for 3 hrs or overnight, then sprinkle with a few cherries and a dusting of icing sugar. Serve with crème fraîche.