This chocolate version of the Italian favorite contains cocoa, miniature chocolate chips, and chopped nuts.
Preheat oven to 375 degrees F (190 degrees C).
Grease baking sheets or line with parchment 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 9x2x1 inch loaves.
Place onto baking sheet 4 inches apart.
Brush with mixture of water and yolk.
Bake for 20 to 25 minutes in the preheated oven or until firm.
Cool on baking sheet for 30 minutes.