Beat butter until soft. Add sugar and baking powder. Beat in eggs and vanilla.
Add cocoa powder and flour to this mixture.
Add the choco chips and nuts.
Divide dough into 2 equal parts.
Shape into small loaf like shape.
Place on baking sheet 4 inches apart.
Brush with mixture made of egg yolk and the water.
Bake 190 degrees Celcius for 25 mins.
Cool for about half an hour.
Cut this loaf shaped baked dough into thick slices diagonally(like biscotti shape).
Lay these cut slices on the baking sheet and bake at 165 degrees celcius for 15 mins.
Turn over the slices to the other side and bake again for 10 - 15 mins until dry and crusty. Cool.
Store in air-tight containers.