Heat the milk, sugar and a large pinch of salt in a medium saucepan. Scrape the seeds from the vanilla bean into the milk with a small sharp knife, then add the bean pod to the milk. Cover, remove from the heat, and leave to infuse for one hour.
Place a 2-litre bowl in an even larger bowl that’s been partially filled with ice. Set a strainer over the top of the smaller bowl and add the cream into that bowl.
In a separate medium bowl, stir the egg yolks together. Gently reheat the milk, then gradually pour a little of the milk into the yolks, whisking constantly until combined. Scrape the warmed yolks and milk back into the saucepan. Cook over a low heat and stir constantly, scraping the bottom with a rubber spatula until the custard thickens enough to coat it.
Carefully strain the custard into the heavy cream and stir over the ice until cool. Stir in the vanilla extract, cover and then refrigerate for 2-3 hours to chill thoroughly. When chilled, remove the vanilla bean and freeze the custard in your ice cream maker according to the manufacturer’s instructions.
While the ice cream churns, brush a 1½ litre loaf tin with vegetable oil, then line it with clingfilm so that plenty hangs over the edges. Add 200g chocolate and the coconut oil to a medium bowl and place in your microwave, heating in 30-second bursts until melted and stirring to combine. Alternatively, place a heatproof bowl over a pan of simmering water and add the chocolate and coconut oil, stirring until melted. Leave the chocolate sauce to cool.
Once the ice cream has churned and the chocolate sauce is no longer hot, get ready to layer up. Working quickly, add a quarter of the ice cream mixture to the loaf tin, spreading it flat. Add a thin layer of the chocolate sauce, spreading carefully with an off-set palette knife. Sprinkle over 1 tbsp of the chopped hazelnuts and then transfer the loaf tin and bowl of ice cream to your freezer for 5 minutes to prevent it melting.
Remove the loaf tin and the bowl of ice cream from the freezer and repeat the above until you have three layers of the chocolate sauce, finishing with a fourth layer of the ice cream. Cover loosely with clingfilm and transfer to your freezer. Leave to set, preferably overnight. Transfer the remaining chocolate sauce to a small bowl, cover and leave in the fridge.
Place a small plate or baking tray in the freezer. Melt the remaining chocolate in the microwave, stirring until smooth. Spread thinly on the plate or baking tray and put back in the freezer. Leave until firm, then remove from the freezer and create curls using a knife or break up into shards. Place back into the freezer.
When you’re ready to serve, gently reheat the chocolate sauce and leave to cool. Remove the Viennetta from the freezer and use the overhanging cling film to pull it out of the loaf tin and turn it over on to a board or serving dish. Drizzle with the chocolate sauce and then, working quickly, before the sauce fully sets, sprinkle the remaining nuts down the centre and decorate with the chocolate curls or shards.