Very rich and more-ish, but easy to make in a food processor.
Preheat the oven to 180°C/350°F/Gas Mark 4.
Put the flour, sugar, cocoa and salt into the bowl of a food processor, and process for a few seconds just to combine the ingredients.
Add the diced butter and process again. After about 30 seconds the mixture will look like fine crumbs; after another 30 seconds or so it will start to come together to make a firm dough. Turn off the machine, remove the blade and tip the dough onto a work surface. Add the nuts and quickly work into the dough. Transfer the dough to the prepared tin. Press the dough into an even layer using the back of a spoon. Prick the dough well and score into 12 sections with a round-bladed knife.
Bake in the heated oven for about 20 minutes until just firm. Remove from the oven, sprinkle with icing sugar then cut into sections along with the marked lines. Leave to cool before removing from the tin. Store in an airtight container. Best eaten within a week.
As the mixture is rich in cocoa watch it carefully as it bakes – you don’t want it to scorch but undercooked shortbread isn’t good either so reduce the oven temperature if it begins to brown before the shortbread is cooked.