Preheat the oven to 150°C / 300°F / Gas mark 2. Line the base and sides of a 20×20-cm/8×8-in brownie tray or baking dish with baking paper, cutting slits in each corner so it fits neatly. Let the paper hang over the edges so you can use it as handles to lift the cooked shortbread out of the tray.
Beat the butter and orange zest together in a stand mixer or in a bowl with electric beaters until pale and creamy –– this will take a good five minutes. Gradually beat in the icing sugar.
In a separate bowl, using a fork or balloon whisk, whisk both flours, the caraway seeds and salt together. Add to the butter mixture and beat on low speed until the flour is only just combined. Don’t overbeat or the shortbread will be tough.
Press the mixture evenly into the prepared tray and bake for 50–55 minutes until firm and the palest gold. Leave in the tray to cool completely.
When cool, lift out onto a chopping [cutting] board. You can cut the shortbread into whatever size pieces you like, but this is the way I like to do it. Using a serrated knife, slice the shortbread square in half and then cut each half into six fingers, roughly 10 × 3cm / 4 × 1¼in. The pieces might crumble, but this is the nature of shortbread –– we’re not aiming for diamond-cut edges here. Transfer to a wire rack set over a rimmed baking sheet or board.
Melt the chocolate in a heatproof bowl set over a bowl of barely simmering water, making sure the bottom of the bowl doesn’t touch the water. (Alternatively, melt the chocolate while keeping it in temper using the hairdryer method.)
Spoon the chocolate over half of each shortbread finger. Set for at least one hour.
Cocoa by Sue Quinn (Quadrille £25) Photography: Yuki Sugiura