Line a 18x27cm (7x11in) rectangular cake tin with nonstick baking paper. Heat the oven to gas 4, 180˚C, fan 160˚C.
Place the butter and sugar in a bowl and beat with a wooden spoon until light and creamy. Beat in the flour until it forms a soft dough. Press the mixture evenly into the prepared tin. Bake for 20-25 minutes until pale golden.
Place the butter, sugar, golden syrup and condensed milk into a large pan over a medium heat. Stir continuously until the sugar has dissolved then turn up the heat and boil for 5-7 minutes, stirring all the time to ensure the mixture doesn't burn on the bottom of the pan.
The mixture is ready when it turns a deep golden colour and has thickened to a soft fudge consistency. Stir in 1tsp salt, then pour the caramel over the shortbread and leave to set for an hour or until chewy.
Put the chocolate in a bowl and melt in the microwave for 1-2 minutes (or place over a pan of simmering water). Pour melted chocolate over the caramel. Leave in a cool place to set, then cut into squares. Keep in a tin for up to 1 week.