Line a 20cm square cake tin with parchment paper.
Place a medium heavy-based pan over a medium-low heat.
Add the cream, sugar and corn syrup and stir continuously with a wooden spoon until it reaches 125ºC (hard-ball stage) usually 20 - 30 minutes.
Remove pan from the heat, then add the butter, vanilla and 2 teaspoons of the salt.
Return to the heat and keep stirring until the butter has melted.
Remove from the heat, pour the mixture into the prepared cake tin and then sprinkle with the last teaspoon of sea salt.
Leave to cool and set, then cut in squares and wrap in small was paper square. Store in an airtight container for up to 1 month.