Preheat oven to 190 degrees C. Grease baking trays or line with baking paper.
In a large bowl, cream together the butter and sugar until smooth. Beat in the eggs one at a time, then stir in the vanilla. Combine the flour, cocoa and baking powder; stir into the creamed mixture until well blended. Dough will be stiff so mix in the last bit by hand. Mix in the chocolate chips and walnuts.
Divide dough into two equal parts. Shape into 20x5x2cm loaves. Place onto baking tray 10cm apart. Brush with mixture of water and yolk.
Bake for 20 to 25 minutes in the preheated oven or until firm. Cool on baking tray for 30 minutes.
Using a serrated knife, slice the loaves diagonally into 2cm slices. Return the slices to the baking tray, placing them on their sides. Bake for 10 to 15 minutes on each side or until dry. Cool completely and store in an airtight container.