Beat butter in a large mixing bowl with an electric mixer on medium to high speed for 30 seconds. Add the 1 cup granulated sugar and baking powder; beat until combined. Beat in eggs. Beat in cocoa powder, espresso powder or coffee crystals (if using coffee crystals, first dissolve in 1 teaspoon water), and as much flour as you can with the mixer. Stir in remaining flour, chocolate pieces, and nuts with a wooden spoon. Divide dough in half.
Shape each portion into an 11-inch-long loaf. Place loaves 5 inches apart on a lightly greased cookie sheet. Flatten each loaf until 2-1/4 to 2-1/2 inches thick. Combine egg white and 1 teaspoon water. Brush on loaves. Sprinkle with coarse or granulated sugar.
Bake in a 350 degree F oven for 25 minutes or until firm and a toothpick inserted in center comes out clean. Cool on cookie sheet for 30 minutes or until completely cool.
Transfer to a cutting board, and cut each loaf diagonally into 1/2-inch-thick slices. Lay slices, cut sides down, on the cookie sheet. Bake in a 325 degree F oven for 8 minutes. Turn slices over and bake for 5 to 10 minutes more or until dry and crisp. Transfer slices to wire racks to cool. If desired, dip part of each slice in melted white baking pieces. Makes about 36.