Grease and line an 8" (20cm) square pan with parchment paper, leaving the parchment extending over the sides of the pan to form a sling to remove the caramel with later. Secure with binder clips if desired.

In a medium sized heavy bottom saucepan, combine the butter, condensed milk, brown sugar, and golden syrup.

Place over medium high heat, stir until melted together, and bring to a boil. Cook, stirring constantly (I like to use a whisk then switch to a rubber spatula) until the caramel registers 240°f / 120°c on a thermometer. Watch the bottom carefully to avoid the mixture catching, turning down the heat if needed or removing the caramel from the heat, stirring well, then returning to the heat.

Once the caramel has reached the correct temperature, remove from the heat, and add the salt and vanilla. Mix well to combine, then transfer to the prepared pan. Shake to evenly distribute the caramel.

Chill the caramel for at least two hours, until completely set and you can no longer feel the heat in it.

Remove the caramel slab from the pan using the parchment overhang. Cut into pieces using a very sharp knife.

Either wrap in wax paper and store in an airtight container in the fridge, or place into an airtight container with wax paper between the layers and store in an airtight container in the fridge. Keeps in the fridge for up to a month.