Preheat the oven to 150C & line a 9in square cake tin with baking parchment.
Combine the flour, caster sugar and butter in a food processor and pulse until the mixture resembles fine breadcrumbs. (Alternatively, you can rub the butter in by hand.)
Tip the mixture into the lined cake tin and spread it out evenly with the back of a spoon and press down firmly so that it is tightly packed in the tin. Dock with a fork.
Bake the shortbread for 30 minutes or until a light golden brown. Set aside to cool.
For the topping, heat the butter, condensed milk, golden syrup & brown sugar in a saucepan, stirring occasionally until the butter is melted. Increase the heat slightly and bring the mixture to the boil, stirring frequently. The caramel will thicken and turn golden-brown, approximately 6 mins. Pour over the cooled shortbread – be VERY careful as the Caramel will be VERY hot. Allow to cool completely.
Melt the chocolate & butter in a bowl set over a pan of simmering water, stirring occasionally.
Pour the melted chocolate over the caramel and set aside until the chocolate has cooled completely.