Place the sweetened condensed milk, butter, brown sugar, golden syrup and salt in a medium saucepan and heat over medium until melted, stirring regularly. Continue to simmer until the mixture has thickened and is golden, 5-6 minutes, stirring constantly to prevent scorching. When you run a spoon down the middle it should leave an empty trail along the saucepan. For accuracy we recommend using a candy or instant read thermometer
. The caramel mixture is done when the thermometer reaches 225 degrees F. Remove from heat and stir in the vanilla extract.