Preheat oven to 375 degrees F. Line baking sheet with parchment paper.

In a large bowl, using a hand mixer, cream together the butter and sugar until smooth. Beat in the eggs and vanilla.

Combine the flour, cocoa and baking powder and stir into the creamed mixture using a wooden spoon, until well blended. Mix in the chocolate the chopped pecans

Divide dough into two equal parts. Shape into 9x2x1 inch loaves. Place onto the prepared baking sheet 4 inches apart.

Mix the water and egg yolk together until well blended. Brush the biscotti log with the egg wash.

Bake for 25 minutes in the preheated oven, or until biscotti is firm to the touch.

Remove from the oven and cool for 5 minutes on the baking sheet.

Using a serrated knife, slice the loaves in a diagonal direction cutting in 1 to 1-1/2 inches wide.

Return the slices to the baking sheet, placing them on their sides. Bake for 15 minutes, or until dry, turning the biscotti over halfway through.

Place on a cooling rack and cool completely. Store in an airtight container.