Beat butter until soft and add sugar and baking powder. Beat in eggs and vanilla, add cocoa and as much flour as you can by mixer and the rest by hand when the dough becomes too stiff. Stir in chocolate chips and nuts.

Divide dough into two equal parts. Shape into 9 x 2 x 1 inch loaves. Place on a baking sheet 4 inches apart. Brush with mixture of water and yolk. Bake for 25 minutes. Cool on sheet for 30 minutes. Cut loaves into 1/2" thick slices diagonally (see photo). Lay cut side down and bake at 350°F (165 degrees° C) for 15 minutes. Turn over and Bake other side for 10 to 15 minutes until dry. Cool. Store in airtight container. (Photo shows a different type of biscotti, but the technique is the same.)