Make the shortbread biscuit base. Using a wooden spoon, beat the butter and sugar together until light and creamy, but not aerated. Sift in the plain flour and cornflour and mix to a dough.
Tip the dough onto a sheet of baking paper and place a second sheet of paper on top. Roll out the dough to a rectangle the same size as the base of the loaf tin, trimming to fit, if necessary. Carefully lift the shortbread dough into the tin, then chill it for 20 minutes.⏱ 20 min
Heat the oven to 170°C/150°C fan/325°F/Gas 3.
Prick the chilled dough all over with a fork and bake it for 15–20 minutes, until a pale biscuit colour. Remove the shortbread from the oven and leave it to cool and firm up in the tin for 5 minutes.⏱ 20 min
Using the baking paper to help you, remove the shortbread from the tin. While it's slightly warm, cut the shortbread into 10 equal fingers (about 1.5cm wide), then set them aside to cool completely while you make the caramel.
Make the caramel. Tip the caster sugar and liquid glucose into a small heavy-based pan and heat the mixture over a low heat until the sugar begins to melt, shaking the pan from time to time (do not stir). Bring the mixture to the boil, then cook the caramel to a light amber caramel colour (about 2–3 minutes). Meanwhile, prepare a bowl of ice-cold water.⏱ 3 min
Carefully remove the pan from the heat and plunge the base into the cold water to stop the cooking process and prevent the caramel from burning.
Add the cream and butter to the pan and stir until the butter melts. Tip the caramel into a clean bowl and leave it to cool (about 1 hour), then chill it until thick enough to pipe.⏱ 60 min
Spoon the chilled caramel into the medium piping bag fitted with the medium plain nozzle and pipe a line of caramel on the top of each shortbread finger. Set aside.
Make the chocolate coating. Melt the chocolate in a heatproof bowl set over a pan of barely simmering water, stirring occasionally until smooth.
Remove the bowl from the heat and pour about one quarter of the chocolate into a shallow bowl. Dip the base of each caramel biscuit finger into the chocolate to coat, then place them on the lined baking sheet. Chill the fingers for 5 minutes, until the chocolate sets.⏱ 5 min
Spoon 2 tablespoons of the chocolate into the small piping bag fitted with a small plain nozzle and set aside.
Pour the remaining chocolate into a jug. Place the biscuit fingers on a cooling rack set over a sheet of baking paper to catch the drips, then pour the chocolate in the jug over the caramel to evenly coat the biscuits. Leave the chocolate coating to set for 2 minutes, then pipe thin lines across the top of the biscuits in a zig-zag motion. Leave the chocolate to set completely (about 15 minutes) before tucking in.⏱ 15 min