Pre-heat the oven to 170°C, gas mark 3
Add the flour, baking powder, cocoa powder and salt into a large bowl and mix well.
Add the ground almonds and whole pistachios and sugar and mix all together
Add the egg and mix it together first with a wooden spoon and then using your hands to bring the mixture together to form a smooth dough. It will be fairly tough dough, but that's fine.
Place the dough on a lightly floured surface and, using your hands, roll it into a log about 30cm long and about 5 cm wide (or similar size to that)
Put the biscotti log on the lined baking sheet and bake it near the centre of the oven for 30 minutes.
After that, transfer it to a cooling tray and leave until completely cold. Don't be tempted to cut the biscotti before they are completely cool otherwise, they will look squished and lose their shape.
Reduce the oven temperature to 150°C, gas mark 2.
Use a sharp serrated knife to cut the biscotti into slightly diagonal slices about 1cm wide.
Place them back on the lined baking tray - this time flat - and bake for another 30 minutes until pale gold and crisp. At this point, you are really just drying them out.
Transfer the bisctotti to the cooling tray and when cold, store in an airtight container for up to week (or more). I've successfully stored my biscotti for about 4 weeks, but they are usually gone by week three.
If you want to, you can freeze biscotti on the day when you bake them and should be fine in freezer up to 1-2 months.