Blend the hazelnuts in a food processor until finely ground. Add the flour, icing sugar and butter. Mix until the mixture comes together. Add the egg yolk and cold water, and mix until the dough comes together. Flatten the dough into a disc, then wrap in cling film and chill in the fridge for 30 minutes.
Preheat the oven to 180 C / Fan 160 C / Gas 4.
Dust your work surface with a bit of flour. Roll out the pastry to line a 23cm tart tin. Transfer the pastry to the tin and trim the edges. Prick the base with a fork, then line with baking parchment and fill with dried beans (you can use rice or lentils as a substitute).
Bake for 20 minutes, or till just golden, then carefully remove the beans and parchment. Bake for 5-10 minutes more or until it is light golden. Leave this to cool.
Place the caster sugar and water into a small pan. Heat gently to dissolve the sugar. Increase the heat and cook until the sugar turns to an amber-coloured caramel. Reduce the heat and add the butter, cream and golden syrup, and stir until the sauce is smooth and thickened. (If the sauce has not thickened, continue to stir the mixture on low heat. Pour into a jug and mix until it thickens.) Add sea salt.
Place in the fridge for 30 minutes to cool, then pour over the cooled pastry case. Place this in the fridge for another 30 minutes.
Melt the chocolate and butter in a heatproof bowl set over a pan of simmering water. Stir until smooth, then remove from the heat and allow to cool slightly. In a separate bowl, whisk the eggs, egg yolk and caster sugar. Fold in the melted chocolate and cocoa, then pour into the tart on top of the caramel. You can leave the tart in the tin or place on a baking tray.
Bake in the oven for 25 to 30 minutes, or until a crust has formed. Leave it to cool before serving.
You can serve your tart as above or you can make this recipe a showpiece for the dinner table. Using the double cream, add icing sugar and whip the cream to a thick consistency and leave to the side.
Melt 50g milk chocolate with 1 tablespoon of water in the microwave, mix this together (it may be thick; if so, add 1 to 2 tablespoons of the whipped cream to form a smooth paste). Leave this in the fridge for about 20 to 30 minutes so it becomes cool. With the remaining whipped cream, spread evenly on top of the tart.
Cut up strawberries into small cube pieces and sprinkle on top of the cream. Also sprinkle on top the chopped hazelnuts for a nutty finish.
Check that the milk chocolate in the fridge is cool and you can either spread it over with a fork or use a piping bag for a neat finish.