Method

1. Heat oven to 160C/140C fan/gas 4. Line a 20cm x 30 cm tin with baking parchment. Rub the flour and butter together with your fingertips until the mixture resembles fine breadcrumbs then stir in the sugar, vanilla and pinch of salt. 

2. Tip the mixture into the prepared tin in an even layer, then press the mixture down firmly with the back of a spoon. Bake for 45-50 mins or until light golden brown. Leave to cool completely in the tin. 

3. For the caramel filling, heat the butter, condensed milk, golden syrup and sugar in a pan over a low heat, stirring occasionally until the butter is melted and the mixture is smooth and glossy. Increase the heat a little so that the mixture starts to bubble, stirring all the time and scraping the bottom of the pan so it doesn’t stick and burn. Keep cooking like this, stirring all the time with a wooden spoon or a metal whisk, for 3-4 mins or until golden (the colour should be somewhere between fudge and toffee). Be careful as the caramel will be very hot. Leave to cool for 5 mins, then pour the caramel over the cooled shortbread. Leave to cool completely.

4. Stir the dark chocolate and the butter in a bowl set over a pan of simmering water until melted (alternatively heat in the microwave, stirring after every 30 sec blast to make sure it doesn't burn). Once melted and glossy pour the chocolate mixture over the caramel. To decorate, go to the next step, or leave to cool completely then cut into squares or triangles.

5. For extra flourish, put melted white chocolate into a piping bag and pipe straight lines over the full length of the shortbread after applying the dark chocolate layer. Turn the tray 90° and drag a skewer through it in long strokes up and down the rows to create a feathered design. Gently shake the tin to settle the chocolate then leave to set completely. Cut into squares or triangles.