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To make the pastry, whizz the hazelnuts in a food processor until finely ground. Add the flour, icing sugar and butter, and pulse until the mixture resembles breadcrumbs. Add the egg yolk and 1-2 tbsp cold water, and pulse until the dough comes together. Tip the dough out and flatten into a disc, then wrap in cling film and chill for 30 mins.
Heat oven to 180C/160C fan/gas 4. On a floured surface, roll out the pastry to line a 23cm loose-bottomed, deep tart tin. Trim the edges and prick the base with a fork, then line with baking parchment and fill with baking beans. Bake for 20 mins, then carefully remove the baking beans and parchment and bake for a further 5-10 mins until light golden. Allow to cool.
Meanwhile, make the salted caramel. Tip the sugar into a small pan, add 1-2 tbsp water and heat gently to dissolve the sugar. Increase the heat and cook until the sugar turns to an ambercoloured caramel. Reduce the heat and add the butter, cream and golden syrup, and stir until the sauce is smooth and thickened. Remove from the heat and add the salt. Allow to cool for a few mins, then spread onto the tart base. Scatter with chopped hazelnuts and set aside.
For the chocolate filling, 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 for about 6 mins until thick and pale. Fold in the melted chocolate and cocoa, then pour into the tart case. Transfer to a baking sheet and cook for 20-25 mins or until set and the top has formed a crust. Allow to cool to room temperature before serving in slices.