Line one large baking tray or two smaller baking trays with parchment paper and leave to the side
Mix together the plain flour, caster sugar and baking powder in a large bowl until combined well
Add in the lemon zest and mix through
Gradually mix in the beaten eggs a bit at a time and mix fully before the next addition of eggs - your mixture shouldn't be sticky (however, if you find it is very very dry and not mixing well then add a touch more egg from a 3rd!) I used my hands to mix
Add the chocolate and nuts and knead in
Turn the dough out onto a lightly floured surface and knead a few times so its smooth
Roll the dough out to form a thick but flat log (I recommend 5-6cm in width, 2cm in depth)
Bake in the oven for 30 minutes
Once baked, remove from the oven, keep them on the trays, and keep the oven on but reduce it to 160ºc / 140ºc fan
Leave the logs for 10-15 minutes until they have cooled slightly
Cut the logs into the biscotti shapes, about 1.5-2cm in thickness and line them up again on the trays with the cut side up and bake in the oven for 10-15 minutes until they are dry through
Remove from the oven and leave to cool
Mix the icing sugar with lemon juice to a thick paste and drizzle over and sprinkle with lemon zest to decorate
Notes
These last for about 2 weeks in an airtight container, but mine never last that long.