Heat the oven to the maximum temperature, or to 250°C.
Crack two whole eggs and an egg yolk into a bowl. Keep the extra egg white to the side, you will need this later on. Whisk, using a planetary mixer or an electric whisk, the eggs with the sugar until you have a light and fluffy mixture.
Then add the sieved flour with baking powder and salt before adding the grated orange rind. Lastly, add the pistachios and chopped chocolate and mix together until it is all combined.
Line a baking tray with parchment paper and, with the help of a spatula, create two loaves with the dough, measuring around 30cm in length and not more than 5cm wide.
Beat the egg white with a fork until it becomes foamy, then brush it over the loaves.
Turn the oven down to 180°C and bake the biscuits. The extremely high temperature ensures that the biscuits won’t lose their shape too much when they start to bake.
Bake the biscuits for around 20-25 minutes, until they are golden outside, but still slightly wobbly inside. If you push down on the surface, it should be relatively hard, but still have a little give.
Leave the biscuits to cool down for around 5 minutes, then cut them on a chopping board, with a sharp knife. The biscuits should be about 2cm thick.
Lay the biscuits out on the baking tray cut side down and bake for another 15-18 minutes, until they have browned. Leave them to cool completely on a cooling rack, then keep them in a biscuit jar or a tin. They should keep for weeks.