Pre-heat the oven to 175°C.
In a food processor blend the flour and salt, add the butter and blitz until you have a breadcrumb consistency. Do not over mix.
Remove from the blender and put in a mixing bowl. Slowly pour the water in, a tablespoon at a time and mix until you have a dough ball. You may not need to use all the water. Wrap in cling film and refrigerate for two hours.
Remove the pastry from the cling film and roll out on a lightly floured surface to 1/4cm inch in thickness. Line an 27cm tart ring with the pastry and place it on a baking sheet. Prick the pastry with a fork and place the lined tart ring in the fridge for 20 minutes. Line the pastry with greaseproof paper and fill with baking beans to keep the base's shape, and bake blind for 20 minutes. Remove the baking beans and greaseproof paper and return to the oven for a further five to eight minutes or until the pastry is starting to turn golden brown. Remove from the oven, brush with egg yolk and allow to cool.
Pre-heat oven to 160°C.
Melt the chocolate with the butter in a Bain Marie, when a good way to being melted add the espresso powder and cocoa powder and stir until smooth. Mix the sugar with the eggs. Add the chocolate mixture and sift in the flour. Lightly toast the crushed hazelnuts in a dry frying pan for a couple of minutes and add to the mixture.
Pour the mix into the tart case and bake for around 40-45 minutes, so that it's not too firm but not too liquid.
Allow to cool before serving.