TO PREPARE THE DOUGH:

1. In the bowl of a stand mixer fitted with a paddle attachment, beat the butter and brown sugar until well incorporated.
2. Add the vanilla and continue beating on speed 2 until the butter is creamy and light, about 4 minutes.
3. Stop the mixer and scrape down the sides with a spatula.  
4. With the mixer set on speed 4, add the eggs one at a time, stopping the mixer occasionally to scrape down the sides.  
5. Continue to beat the butter mixture until the sugar breaks down and develops a smooth silky texture, about 10 minutes.  To test, rub the butter mixture between your thumb and index fingers.  It should feel smooth with very little abrasiveness from the sugar. 
6. Stop the mixer and add the flour, cocoa powder, salt, baking powder, and espresso powder.  I highly recommend sifting the cocoa and baking powders through a flour sifter or fine sieve.  This step will help to eliminate any clumping, especially if you live in a high humidity climate. 
7. Using a spatula, gently hand mix the flour into the butter mixture, just until the flour starts to blend into the butter.  (This step will reduce the loss of flour exploding into the air.)
8.  With the mixer on low speed continue to beat just until the flour is thoroughly combined.  
9. Stop the mixer to scrape down the sides.   
10. With the mixer set on low speed add the toasted almonds and chocolate pieces, beating until just combined, about 1 minute.
11. Remove the paddle and remove the bowl from the mixer.
12. Divide the dough into two halves, approximately 440 grams each and place each half on a sheet of plastic wrap.   Side Note:  This dough is very dense with a fudge-like texture causing some of the chocolate pieces and nuts to fall out.  It may be necessary to redistribute these strays by pushing them back into the dough.  
13. Form each dough ball into a log using your fingertips slightly dampened with water.  The log should be about 10 inches long by 2 1/2 inches wide and 1 inch in thickness.  Wrap each log in the plastic wrap and continue to shape the sides of the log by gently dropping each side onto a flat surface.  Secure the ends of the plastic wrap and chill for one hour.  (This cookie dough will keep for 3-4 days refrigerated, should you prefer to bake the biscotti at another time.

BEFORE BAKING:

1.  Preheat the oven to 350 degrees F.
2. Line a baking sheet with parchment paper or a Silpat.
3. Make egg wash by placing both the egg and water in a glass pyrex.  Whisk to combine.  Alternatively,  I prefer to mix my egg wash in a small glass lidded jar by shaking the egg and water together until combined.   (Then it can easily be stored in the refrigerator for a day or two, if I am going to be baking more cookies.  Additionally,  the leftover egg wash can be utilized in my next mornings breakfast scramble, eliminating any waste.) 
4. Unwrap each cookie log and place on the prepared baking sheet at least 3 inches apart from each other.
5. Brush each log, both the top and sides, with egg wash.  (If you desire to make a prettier finished biscotti, sprinkle the top of each log with coarse white sugar or raw sugar.  However this step is optional.)
6. Bake until the tops are cracked and firm, 30 to 35 minutes or a digital thermometer registers 190 degrees F.
7. Remove from oven and let logs cool on pan for 10-15 minutes.  Then transfer to a cooling rack for at least 30 minutes.  Note:  I have found that letting the cookies cool for a few hours before cutting helps to eliminate the smearing of melted chocolate.  This step isn’t necessary, it is strictly for aesthetics.
8. Reduce the oven to 250 degrees F.  And transfer the logs to a clean cutting board (one without lingering onion odors).
9. Using a serrated knife, gently cut the logs crosswise on a slight diagonal into 1/2 inch slices. (See pictures above.) 
10. Place the cookie slices, cut side down, on the same baking sheet.  (It is helpful to place the biscotti facing the same direction.   When it is time to turn the cookies over to bake the other side, there is no confusion of which cookies have been turned over.)
11. Place in the reduced-heat oven and bake for 15 minutes.  Remove from the oven and let cool for 5 minutes before turning over to the other cut side.
12. Place baking sheet back in the oven and bake an additional 12 minutes.   Remove from oven and let cool for 5 minutes.
13. Transfer biscotti to a cooling rack and allow to cool completely before storing.
14.  Store biscotti in a sealed air-tight jar.  Biscotti may also be frozen by placing in a heavy-duty freezer bag before freezing.  Simply defrost to room temperature and serve.