Cook over a low heat, stirring constantly until the custard has thickened enough to coat the back of a spoon, don't allow it to get too hot or it will scramble. Pour the custard through the sieve into the cream and whisk together. Whisk in the vanilla and salt. Cover and refrigerate until completely cold. (I refrigerate it overnight).
While the custard is chilling make the praline. Preheat the oven to 150°C/300°F/gas 2. Spread the hazelnuts on a baking tray and roast for about 20 minutes, stirring every now and again, until richly golden.
Place the hazelnuts in a clean teatowel and rub to remove the skins. It doesn't matter if you don't get all of it off.
Line a heatproof tray with some baking parchment. Put the sugar and water in a small saucepan. Bring to the boil and swirl the pan (don’t stir) until the sugar dissolves. Keep cooking until the caramel turns an amber colour. Remove from the heat and tip in the hazelnuts and a pinch of salt, stir until they are covered in the caramel. Pour the praline on to the prepared tray and let it cool completely.
Once the ice cream is churned, fold through about 2/3rds of the praline. Transfer to a container and sprinkle over the rest of the praline. Place in the freezer for a couple of hours before serving.