Heat oven to 180C/160C fan/gas 4. Put the flour, cocoa, ground almonds, sugar, chocolate chips, candied peel, orange zest and a pinch of salt into a large bowl, and mix well. Make a well in the centre, then add the egg and vanilla essence. Use your hands to bring the mix together, kneading it firmly.
Shape the biscotti dough into a log approximately 15cm x 5cm x 2.5cm deep. Transfer to a baking tray lined with baking parchment, then bake for 30 mins until firm to the touch (the log will split and crack a little on the top).
Remove from the oven and cool for at least 10 mins. Use a sharp serrated knife to cut the log into slices 1-1.5cm thick.
Put the biscotti back on the baking tray and reduce oven to 150C/130C fan/ gas 2. Bake for 30 mins or until the biscotti are completely hard to touch. Cool completely, then wrap in baking parchment or store in an airtight container.