Make Caramel - In a medium saucepan over medium heat, cook condensed milk, butter, golden syrup, brown sugar, and salt. Once everything is melted, cook another 5-6 minutes, stirring constantly, until it has thickened and turned golden. When an instant read thermometer reads 225°F (125°C), remove from heat and stir in vanilla extract. Pour caramel over shortbread and smooth it out with the back of a spoon. Let it cool completely.