Preheat the oven to 180C / 350F / 160C fan forced. Line a large baking tray with baking paper.
Place the flour, sugar, cocoa, baking powder, salt and 1/4 cup of chocolate chips in the bowl of a food processor. Process until it looks like crumbs.
Add the eggs and vanilla and pulse until it starts to clump.
Tip the dough out onto a surface dusted with cocoa. Work half the pistachios through without kneading it too much but just enough to pull it together into a disk.
Divide the dough in two and flatten out to about 18cm x 10cm (7×4 inch). Transfer them to the baking tray and bake for 30 minutes until dry on the outside.
Let the logs cool for 5 minutes before cutting them into 1cm thick pieces (notes).
Now stand the biscotti on their edges back on the same baking tray and bake for a further 15 minutes. They will look very dry but will still be the slightest bit soft.
Once the biscotti has cooled, melt the remaining chocolate chips in a small bowl over simmering water. Use a spoon or a squeeze bottle to drizzle the chocolate over, then sprinkle with the remaining pistachios