Place the cornflour in a saucepan and whisk in a splash of the milk, making sure there are no lumps. Whisk in the rest of the milk, the sugar, vanilla paste and a pinch of salt.
Heat this mixture, stirring constantly, until it comes up to the boil and thickens. Remove from the heat, allow to cool slightly then whisk in the oil and cream.
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.
Churn the chilled custard in an ice cream machine according to the manufacturers instructions. Meanwhile, break the cooled praline into pieces and place in a sturdy food bag. Bash with a rolling pin until it is broken up into small pieces. Alternatively you can pulse it in a food processor.
Once the ice cream is churned, fold through about ⅔rds 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.