Line a 20cm square tin with foil and rub generously with vegetable oil. Add half the cashews to the tin. In a saucepan, bring the butter, cream and vanilla to the boil, then remove from the heat.
In a large heavy-based saucepan, heat the sugar and syrup on a medium-low heat until it reaches 155C on a sugar thermometer. Do not stir or the sugar will crystallise. Turn off the heat and very carefully add to the cream mixture. Stir together and heat again until it reaches 125C on the thermometer. Remove from the heat and pour in the remaining nuts. Quickly pour into the tin, sprinkle over the salt and leave to cool.
When firm (after 3-4 hrs), cut into pieces –use a knife that has been dipped into boiling water. Wrap in baking parchment until ready to eat. Will keep for 2 weeks in an airtight tin.