Please note: you will need a candy thermometer to make this recipe. Getting the precise temperature for your caramel is important to making sure it sets properly.
Spray an 8 x 8-inch / 20 x 20cm square baking tin with oil or brush with melted butter. Line the bottom and sides with baking paper leaving some overhang on the sides.
Making the caramel - To a large heavy bottomed saucepan add the butter, sweetened condensed milk, golden syrup, granulated sugar. Heat over medium low heat, stirring continuously with a wooden or silicon spoon. Use your candy thermometer to check the temperature of the candy. You want to do this by making sure it’s in the center of your pot and not touching the bottom.
Getting to the right temperature - Continue stirring until the mixture reaches a final temperature of 120C / 240F. You want to make sure you keep a close eye on your caramel mixture as it will heat up gradually and then very quickly when it reaches about 110C / 230F which means it can burn easily if you’re not standing over it, watching the temperature.
Cooling and setting - As soon as it reaches 120C / 240F, take it off the heat, add the vanilla extract and stir. It will bubble and steam. Then pour into the prepare tin. Allow to set for one hour at room temperature before sprinkling with sea salt flakes. Continue cooling until it reaches room temperature and then transfer to the fridge to set completely, about 3 hours.
Slicing up - once set, take out of the tin, lifting from the excess baking paper on the sides. Use a large knife to cut the caramels in 4 rows and then 8 rows, giving you a total of 32 chews. You can wrap these up in wax paper cut into 15cm / 6-inch squares. Twist on each end.