Line an 8” square cake pan with non-stick foil (or use regular foil and grease it with butter).
Combine the cream, corn syrup and salt in a medium large saucepan and heat gently over medium-low heat. Add the sugar in 3-4 additions, whisking gently to combine and trying not to splash the mixture up the sides of the pan. Continue stirring until all of the sugar has dissolved. Be patient, it will take around 10 minutes.
Brush down the sides of the pan with a pastry brush dipped in warm water. Increase the heat to medium high, clip on a candy thermometer and cook without stirring until the mixture reaches 114°C/237°F.
Leaving the thermometer in place, remove the pan from the heat and place it on a cooling rack. Drop in the cubes of butter, but don’t stir it yet.
When the thermometer shows that the mixture has cooled to 52°C/125°F, transfer it to the bowl of an electric mixer fitted with the paddle attachment, add the vanilla and beat on low speed until it loses its shine and thickens.It will look a bit slimy to start off with, and then it will look like cake batter. What you want is for it to look more like soft cookie dough.Scrape the fudge into the cake pan and smooth the top with a spatula.
Leave at room temperature for several hours or overnight until firm.
Place the tin onto a heatproof surface and sprinkle the top of the fudge with the extra tablespoon of caster sugar. Shake the tin to distribute the sugar evenly. Heat with a culinary torch until the sugar bubbles and caramelises. Allow the fudge to cool and become firm again.
Turn the fudge out onto a baking paper covered cutting board (leave the fudge caramel side down to make cutting neater) and cut into 1” squares.Store fudge in an airtight container at room temperature for up to one week.
Notes
*If you don't have both vanilla extract and vanilla paste, use three teaspoons of whichever one you have.Troubleshooting:Fudge can be fickle, and the temperature and humidity of the kitchen when you're making it can drastically affect the results of the fudge.
If your fudge doesn't thicken after beating, then you may need to try again and reduce the amount of corn syrup you use (by a tablespoon or so), in order to get a few more crystals forming.
If the opposite happens and your fudge turns out too grainy, then next time increase the corn syrup slightly.
Once you've successfully made the fudge, make sure you write down what you changed (and what time of year / what the temperature and humidity were like) so you'll know for next time.