Preheat oven to 190 degrees C. Line a baking tray with baking paper.
In a large bowl, cream together the butter and sugar. Beat in the eggs one at a time then add the vanilla essence.
Combine the flour, baking powder and cocoa. Stir into the creamed mixture until mixed well. Mix in the chocolate chips and walnuts by hand. The mixture will be stiff.
Divide the dough into two pieces. Shape into loaves, about 20cm x 5cm x 2cm. Place onto the baking tray, 10cm apart. Brush the tops with the egg yolk and water mixture.
Bake for 20 minutes until firm.
Remove the loaves along with the baking paper and place on a wire rack to cool for 10 minutes only, do not cool for longer.
Cut both 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. If you like, once cool drizzle with melted chocolate.