Butter and line the base and sides of a 20 x 30cm tin with baking parchment.
Pour the cream into a saucepan along with the butter and vanilla, then bring to a simmer. Swirl the pan to melt the butter, then remove from the heat.
In a separate pan, melt the sugar and golden syrup together, stirring occasionally. Once the sugar granules have dissolved, bring to the boil and cook until the temperature reaches 155C on a sugar thermometer. Remove from the heat and swiftly whisk through the cream mixture. Return to the heat and cook until the mixture reaches 127C, constantly whisking so it doesn’t catch.
Pour the caramel into the tin, tilting so it reaches the corners. Leave to set for 15 mins, then sprinkle with the salt. Leave to cool completely at room temperature (preferably overnight). Cut into squares using a very sharp, slightly oiled knife, then wrap each one in a square of baking parchment.