Place the butter, syrup, sugar, condensed milk in a medium-sized saucepan and bring to a boil, stirring the entire time all the ingredients are melting together. Once the caramel has started to heat up and boil, whisk/stir constantly for 5 to 10 minutes until the caramel has started to thicken and brown. To test the caramel, take a cold teaspoon or knife and dip it in, if it sets up fairly quickly the caramel is done. Take off the heat and stir through the salt and vanilla.
100 grams light brown sugar, 397 grams sweetened condensed milk, 125 grams butter, 2 teaspoon sea salt, ½ teaspoon vanilla extract, 50 grams golden syrup