To make the pastry, cream the butter and sugar with a wooden spoon or in an electric mixer until light and creamy. Add the egg yolks and 35ml water and mix well until all the liquid has been incorporated. Gradually mix in the flour and cocoa powder until a paste is formed. It will be quite a soft dough.
Wrap the pastry in foil, flatten it with your hand and chill for 1 hr. The pastry can be made two days ahead, though in this case, you may want to remove it from the fridge for a few minutes before rolling to soften it.
Sprinkle your work surface with a light dusting of flour and roll out the pastry until it’s about 5mm thick. Line a 23cm tart tin with the pastry using the rolling pin to help you lift it over, pushing the pastry well into the edges and folding the excess over the top. Trim off the excess. Chill for 15 mins to relax the pastry and to help prevent it from shrinking during cooking.
Heat oven to 180C/160C fan/gas 4. Line the tart with a sheet of baking parchment (scrunch it up first to soften it), then tip in baking beans, rice or lentils and bake blind for 15 mins. Lift out the beans and bake for a further 10-12 mins until the base is dry. Allow to cool.
To make the filling, put all the ingredients in a glass or metal mixing bowl, place it over a pan of very hot water and allow them to melt together until glossy and thick. Pour into the cooled pastry case and chill for 4 hrs or overnight.
For the topping, heat the sugar in a saucepan on a medium heat, tilt the pan rather than stirring, until you have an even golden liquid caramel. Add the salt and mix well with a wooden spoon or heatproof spatula. Pour in the pecans and, mixing thoroughly, quickly pour the mixture on to a piece of baking parchment and spread out with the back of the spoon. Allow to cool thoroughly, then break or chop up into shards or chunky pieces to sprinkle over the ganache.
Cut the tart with a hot, wet knife for perfect, restaurant-quality slices.Recipe adapted from Adventures with Chocolate by Paul A Young (£14.99, Kyle Books). Photographs © Anders Schønnemann.