To make the hazelnut praline paste, place the hazelnuts on a baking tray lined with greaseproof paper and roast in a pre-heated oven at 170C for about 10 minutes until golden and fragrant.
Place the sugar and water in a saucepan and cook over a low heat to dissolve the sugar. Increase the heat to high and cook the syrup to 116C.
Add the warm hazelnuts and stir to coat them evenly. Continue cooking the hazelnuts and syrup until they are a deep amber colour and no sandy white bits remain. Spread onto a baking sheet to cool completely.
To turn this into praline paste, put the caramelized hazelnuts in a food processor, blend until a smooth and runny paste forms this will take about 8 to 12 minutes. This recipe will make more than you need. Place the excess in a sterilised jar.
Next make the pastry. Place the flour and butter in the food processor and gently mix until it resembles breadcrumbs.
Add the icing sugar and vanilla bean paste and mix through.
Add the egg yolk and the milk and mix until it starts to form a dough.Take the dough out of the machine and shape it into a block about 1cm thick, this will make it easier to roll out.
Rest in the fridge for at least 1 hour.
Take a 25cm flan/pie dish and roll out the pastry between two sheets of greaseproof paper so that you don't add any extra flour. When the pastry is rolled out big enough to cover the tin lay the pastry over the tin, pressing it into the sides and base of the tin.
Leave the excess pastry overhanging the edge, this will help with shrinkage. Trim the pastry a little if necessary.
Place the pastry case in the fridge for 30mins to chill a little. Pre-heat the oven to 180C Fan/200C and line the pastry case with baking parchment and baking beans. Blind bake the pastry for 15 minutes. Then remove the baking beans and baking parchment and return to the oven for 5 minutes.
Check the pastry over to see if it needs any of the cracks patching use some raw pastry and fill the holes. Take the egg wash and brush it all over the pastry and bake for another 15 minutes.
When the pastry is golden, remove it from the oven and leave it cool, then take a vegetable peeler and shave off the excess pastry to remove it.
Take a few spoonfuls of the hazelnut praline paste and cover the base of the pastry with an even layer about 2-3mm thick and place the case in the fridge.
Meanwhile, for the filling, bring the cream, vanilla extract and sugar slowly to the boil in a pan.
Break the chocolate and butter into pieces and place into a large bowl. Pour over the cream mixture and whisk slowly until the chocolate has melted and the mixture is smooth.
Pour the chocolate mixture into the tart case, making sure it covers the hazelnut paste completely and place in the fridge to set for two hours.
Finally make the dark chocolate glaze, soak the gelatine in a bowl of cold water for a few minutes until soft. Squeeze out the excess water.
Put the water and sugar in a saucepan, bring to the boil, then continue to simmer over a low heat for 2-3 minutes.
Add the sifted cocoa powder and the cream, mix thoroughly.
Bring back to the boil and simmer for 4-5 minutes.
Take the pan off the heat and add the pre-soaked gelatine and stir until dissolved.
Strain then leave to cool.
Gently pour the glaze over the chocolate tart, until it is covered by an even layer of glaze. Place in the fridge to set for at least an hour.
To serve the tart take a knife and warm it under hot water, dry it with a towel and then use it to glide through the dessert.
Leave the tart to come up to room temperature before serving, so that the flavours really develop.