Depending on how thick you want your shortbread layer, use an 8x8 or 9x9 inch square pan for the Homemade Scottish Shortbread.
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.
Pour the caramel mixture over the prepared Scottish shortbread and use a spoon to evenly smooth it out. Let it cool completely.
Heat the chocolate in a microwave-safe bowl until melted, stirring occasionally. Pour the chocolate over the firmed up caramel and smooth it evenly. Place it in the fridge to cool completely. *You can store it in the fridge but it is best served at room temperature.
Cut the Millionaire's Shortbread into squares (it's easiest at room temperature).