Preheat the oven to 350F/160C.Grease a 9x13 inch pan.
Using a food processor or mixer fitted with the paddle attachment beat the butter, sugar, vanilla and brown sugar vigorously until pale and fluffy, about 3 minutes. It’s an important step to beat the butter and sugar well together when making shortbread as this ensures the shortbread is light, crisp and will hold together.In a separate bowl, sift the flour and salt. Add it to the butter mixture and mix until blended (forming large clumps).
Turn onto a floured surface and using floured hands, press evenly into the prepared pan (including the sides).
Bake for 40 - 45 minutes, until lightly browned.
Leave to cool completely in the pan.
For the caramel
Combine the sugar and syrup in a medium saucepan. Cook on medium heat, stirring constantly until it becomes a light caramel color. Add the butter, salt & milk stir until smooth. Remove from heat and mix in the vanilla.
Set aside to cool slightly, then pour over the cooled shortbread.Allow cooling completely.
For the topping
Melt the chocolate melt chocolate in a microwaveable bowl for 30 seconds at a time (stirring in between) until the chocolate is smooth and fluid.Pour the melted chocolate over the caramel and set aside until the chocolate has cooled completely.
Cut into squares and serve.
Make sure the caramel has set otherwise you might end up with the chocolate and caramel mixing. It’s worth waiting to have the best end product.
For a show stopping billionaire shortbread cookie bar, why not add crushed hard pretzels, toasted nuts or candied ginger to the chocolate topping.