Preheat oven to 190 degrees C. Line baking tray with baking paper.
In a large bowl, cream together the butter and sugar until smooth (I use a mixer.) Beat in the eggs one at a time, and then add the vanilla and peppermint essences.
Combine the flour, baking powder and cocoa. Stir into the creamed mixture until mixed well. As the mixture will be quite stiff at this stage, add the chocolate chips and walnuts and stir by hand until everything is mixed well together.
Divide the dough equally in half. Shape into two 23x5x2.5cm loaves. Place onto the lined baking tray, 10cm apart. Brush the tops with the egg yolk and water mixture. Bake for 20 minutes in the preheated oven until firm.
Remove the loaves along with the baking paper and place on a wire rack to cool for 10 minutes. Do not let it cool for longer, as this will cause crumbly Biscotti.
Cut each of the loaves into 12 slices using a sharp serrated knife. Return the slices to the baking tray (no need to use the baking paper) and bake on each side for a further 10 minutes until dry, taking extra care to make sure they don’t burn.
Leave to completely cool on the wire rack.
Freezing and Presentation Tips
Biscotti will keep for days in an airtight container and freeze well in an airtight plastic bag or airtight container for a few months. If I am planning to eat them pretty much straight away or giving them as a gift, I always wrap each one with cellophane - it helps keep them fresh and they look great.