Chocolate pastry:
Put the sugar and egg yolk in a mixing bowl, combine briefly with a wooden spoon, then add the butter.
Using the spoon or your fingers, blend them until they come together. You can also do this in a freestanding electric mixer.
Sift in the flour and cocoa and quickly work everything into a dough. Shape into a flat slab, wrap in cling film and chill for 1 hour before use. The pastry will keep in the fridge for 5 days and can also be frozen, well wrapped.
Make the pastry, then rest it in the fridge for at least 1 hour, preferably several, or, better still, overnight.
If using tinned or defrosted cherries, drain them and put them into a bowl with the kirsch. Leave to soak overnight.
Lightly grease a loose-bottomed tart tin, approximately 35 x 11 cm and 2 cm deep.
Skim a fine film of flour over your work surface, roll out the pastry 5 mm thick and use to line the tin (see pages 30–34). Line with baking paper and ceramic beans. Place in the fridge to rest for at least 30 minutes.
Meanwhile, make the chocolate fondant. In a mixing bowl whisk the sugar, eggs and egg yolks until they are pale and have a creamy, mousse-like consistency.
Break the chocolate into chunks and put into a heatproof bowl. Place this over a pan of simmering water – you need enough water to come close to the bottom of the bowl but not actually touch it. Turn the heat very low so that you don’t get steam into the bowl, as this can make the chocolate stiffen and look dull. Let the chocolate melt slowly, stirring all the time, then add the butter and keep stirring until it has melted. Take off the heat and add to the sugar and egg mixture, again stirring well until it is all incorporated. Gently fold in the flour and keep to one side.
Preheat the oven to 190ºC/Gas 5.
Remove the tin from the fridge, place on a baking tray and bake for 20 minutes. Lift out the paper and beans, brush the pastry with the beaten egg, then bake for another 10 minutes. Set aside (see pages 37–41).
Lower the oven to 180ºC/Gas 4.
Spread the fondant over the cooled pastry up to the top, then push the cherries into it, leaving some of the fruit showing (if using fresh cherries, leave the stalks sticking out). Put back on the baking tray and return to the oven for 12 minutes, until just set. The fondant will have risen a little, but, like a chocolate brownie, it should be only just firm to the touch as it will set a bit more when it cools. Leave in the tin for about 15 minutes, then lift out and cool on a rack.
The tart can be served either warm and gooey, or completely cold, when the chocolate will be a bit firmer. Dust with cocoa powder before serving.