A crisp, orange-flavoured shortbread base topped with a salted caramel layer, lively orange ganache and a thin disc of tempered dark chocolate.
Make the shortbread. Put the cornflour, plain flour and icing sugar into the bowl of a food processor and pulse until well mixed. Add the butter, vanilla and orange zest and blitz until the mixture forms a firm dough.
Turn out onto a lightly dusted worktop and roll out to a 21cm square, about 8mm thick. Transfer to the lined baking sheet, prick all over with a fork, then chill for 15 minutes. Meanwhile, heat the oven to 180°C/160°C fan/350°F/Gas 4.
Bake the shortbread for about 17–20 minutes, until a light golden brown and just firm. Remove from the oven, set the baking sheet on a wire rack, and use the 6cm cutter to stamp out 9 discs. Leave to cool on the baking sheet.
Cut the acetate sheets into 9 strips each measuring 22 x 4cm. Wrap one strip around the base of each shortbread disc to make a collar, securing it with sticky tape. Place onto a clean baking sheet.
Make the caramel. Put the butter, cream and salt into a small pan and heat gently until almost boiling.
Meanwhile, put the sugar and 50ml of water into a heavy-based medium pan over a low heat. Heat gently, stirring occasionally, until melted. Turn up the heat and boil rapidly until the syrup turns a rich caramel colour.
Remove the caramel pan from the heat, protect your hand, and carefully pour the hot cream mixture into the pan. (Take care: the mixture will splutter.) Return the pan to the heat and stir continuously until the sauce is smooth and thick, and reaches 110°C on the sugar thermometer.
Remove the pan from the heat and leave to cool, stirring frequently, until the caramel has thickened and cooled to 35–40°C. Pour 2 tablespoons of the caramel into each mould, on top of the shortbread. Chill for 20 minutes, until firm.
Make the orange ganache. Pour the orange juice into a medium pan and simmer for about 20–25 minutes until reduced to 50ml. Pour into a small bowl and leave to cool (it will be thick and sticky).
Put both chopped chocolates into a heatproof bowl. Heat the cream until boiling, then it pour over the chocolate. Leave for 1 minute, then stir until the chocolate is smooth. Stir in the sticky orange concentrate.
Remove the shortbreads from the fridge and spoon 2 tablespoons of the ganache on top of each caramel layer. Return to the fridge to set.
To make the chocolate decoration, put the chocolate in a heatproof bowl suspended over a pan of steaming-hot water. Meanwhile, fill the sink with ice cubes and cold water. When the chocolate is very smooth and reaches 44°C on the sugar thermometer, remove the bowl from the pan and plunge the base in the icy water. Stir continuously until it cools to 28°C, then place it over the steaming water again and stir until it reaches 31°C.
Remove the bowl and set it on a heatproof surface. Stir frequently until the chocolate is slightly cooler and thicker. Meanwhile, cut an 18cm square from the acetate. Using an offset spatula, spread the chocolate in an even layer (2–3mm thick) over the acetate. Leave until nearly set but still flexible, then stamp out 9 discs with the 6cm cutter. Leave to set.
To assemble the shortbreads, peel off the acetate collars and set the shortbreads on a serving plate. Peel the chocolate discs away from the acetate and set one on top of each shortbread. Lightly dust with the orange lustre powder.